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5.7: Greenhouse Gases and the Keeling Curve - Geosciences


The Greenhouse Effect

One of the most common misconceptions about global climate is that the greenhouse effect is just a hypothesis whose role in recent climate change is debatable. The sun's radiation is transmitted to the Earth through our atmosphere. This warms up the surface of the planet and it's an extremely important effect because without it the planet would be way too cold for us to live on it. We've also synthesized some greenhouse gases that aren't naturally occurring and added those to the atmosphere, too.

Credit: Intergovernmental Panel on Climate Change Working Group I.

Even though Earth's natural carbon cycle moves a gigantic amount of carbon between the land, sea, and atmosphere naturally, the balance is pretty delicate and the amount humans have been adding to the atmosphere since the Industrial Revolution lingers in the atmosphere for over 100 years. This means that the effects of climate change we feel now were produced by activities in the past. The fact that greenhouse gases keep being emitted by human activity now means that we have already committed to a warmer future.

Carbon dioxide

The "Keeling Curve" might be the most famous plot of global climate data. Charles Keeling began measuring the atmospheric concentration of carbon dioxide at Mauna Loa in 1958. Today, four air samples an hour are collected from the observation towers at Mauna Loa and the concentrations of several gases are measured. The NOAA keeps track of observations from over 50 stations around the world. The average concentration of CO2 in the atmosphere has steadily increased ever since monitoring began.

Click for a text description of Figure 5.8.

Concentration of carbon dioxide in the atmosphere measured at Mauna Loa Observatory from 1974 to 2008. The blue-shaded data has been quality checked and the grey-shaded data is preliminary. The wiggly lines show seasonal variations in carbon dioxide and the straight line is a running average that effectively removes the seasonal variations.

Credit: NOAA Earth System Research Laboratory

Let's take a look at this plot together (oops! note that in my explanation I mistakenly say that the y-axis is carbon dioxide concentration in millimoles per mole of air when in fact it is micromoles per mole.):

In addition, check out this really cool animation that shows how carbon dioxide concentrations have increased in the atmosphere globally over the past few decades. In the video, the x-axis is latitude and the y-axis is CO2 concentration. The various different symbols represent different types of recording stations (tower, airplane, etc) and the line is an average value.


Unit 5: Modern CO2 Accumulation

These materials have been reviewed for their alignment with the Next Generation Science Standards as detailed below.

Overview

Science and Engineering Practices

Analyzing and Interpreting Data: Construct, analyze, and/or interpret graphical displays of data and/or large data sets to identify linear and nonlinear relationships. MS-P4.1:

Constructing Explanations and Designing Solutions: Make a quantitative and/or qualitative claim regarding the relationship between dependent and independent variables. HS-P6.1:

Analyzing and Interpreting Data: Evaluate the impact of new data on a working explanation and/or model of a proposed process or system. HS-P4.5:

Analyzing and Interpreting Data: Compare and contrast various types of data sets (e.g., self-generated, archival) to examine consistency of measurements and observations. HS-P4.4:

Cross Cutting Concepts

Scale, Proportion and Quantity: The significance of a phenomenon is dependent on the scale, proportion, and quantity at which it occurs. HS-C3.1:

Scale, Proportion and Quantity: Patterns observable at one scale may not be observable or exist at other scales. HS-C3.3:

Patterns: Empirical evidence is needed to identify patterns. HS-C1.5:

Patterns: Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena HS-C1.1:

Cause and effect: Empirical evidence is required to differentiate between cause and correlation and make claims about specific causes and effects. HS-C2.1:

Disciplinary Core Ideas

Global Climate Change: Human activities, such as the release of greenhouse gases from burning fossil fuels, are major factors in the current rise in Earth’s mean surface temperature (global warming). Reducing the level of climate change and reducing human vulnerability to whatever climate changes do occur depend on the understanding of climate science, engineering capabilities, and other kinds of knowledge, such as understanding of human behavior and on applying that knowledge wisely in decisions and activities. MS-ESS3.D1:

Weather and Climate: Gradual atmospheric changes were due to plants and other organisms that captured carbon dioxide and released oxygen. HS-ESS2.D2:

Weather and Climate: Changes in the atmosphere due to human activity have increased carbon dioxide concentrations and thus affect climate. HS-ESS2.D3:

Earth Materials and Systems: The geological record shows that changes to global and regional climate can be caused by interactions among changes in the sun’s energy output or Earth’s orbit, tectonic events, ocean circulation, volcanic activity, glaciers, vegetation, and human activities. These changes can occur on a variety of time scales from sudden (e.g., volcanic ash clouds) to intermediate (ice ages) to very long-term tectonic cycles. HS-ESS2.A3:

Performance Expectations

Earth's Systems: Analyze geoscience data to make the claim that one change to Earth's surface can create feedbacks that cause changes to other Earth systems. HS-ESS2-2:

  • team-based development to ensure materials are appropriate across multiple educational settings.
  • multiple iterative reviews and feedback cycles through the course of material development with input to the authoring team from both project editors and an external assessment team.
  • real in-class testing of materials in at least 3 institutions with external review of student assessment data.
  • multiple reviews to ensure the materials meet the InTeGrate materials rubric which codifies best practices in curricular development, student assessment and pedagogic techniques.
  • review by external experts for accuracy of the science content.

This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are

  • Scientific Accuracy
  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
  • Robustness (usability and dependability of all components)
  • Completeness of the ActivitySheet web page

For more information about the peer review process itself, please see https://serc.carleton.edu/teachearth/activity_review.html.

This page first made public: Jul 15, 2016

Summary

Students will examine data that record the modern increase in carbon dioxide concentrations and the associated increase in average temperatures, and they will investigate the effects of carbon dioxide on various components of the Earth system (atmosphere, cryosphere, hydrosphere — oceans). Students also learn how the burning of fossil fuels contributes to increases in atmospheric carbon dioxide.


Already, Earth has proven quite sensitive

Since the onset of the Industrial Revolution, Earth's average temperature has risen by 1.8 degrees Fahrenheit, or 1 degree Celsius.

Major consequences have already been regularly observed in Earth’s water cycle -- bringing greater odds of extremes in deluges and drought. The most easily-predicted results, record-breaking heat waves and historic wildfires, are manifesting globally, as well as more complex atmospheric changes.

“It [global warming] raises sea levels and makes storm surges worse, it makes the atmosphere wetter, leading to flooding from extreme rainfall, and warming ocean temperatures provide extra energy to tropical storms,” climate scientist Stefan Rahmstorf, head of Earth System Analysis at the Potsdam Institute for Climate Impact Research, said in September.

"The polar ice is melting, in the ocean the Gulf Stream System is weakening, and in the atmosphere the jet stream is getting weird,” Rahmstorf added.

Unlike previous geologic epochs, the defining circumstance today isn’t just notably high carbon in the air -- it’s how fast it’s all accumulating.

The natural world both loads and removes carbon from the atmosphere over long periods of thousands to tens of thousands of years.

For example, a warm period called the Eemian, which ended around 120,000 years ago, slowly melted a significant portion of Greenland’s ice sheets -- even with profoundly lower carbon concentrations of around 280 ppm.

But these days, the climate hasn’t yet caught up.

“We’re warming so fast that we haven’t even begun to let Greenland melt,” noted UC Irvine’s Prather.

Where civilization ultimately ends up, carbon-wise, is contingent upon how quickly global societies transition to clean energy, and generate electricity without a deep reliance on fossil fuels.

“I would argue what's really relevant is where we stabilize out,” said Lachniet. “Over the next hundred years we really set the next 10,000 years of climate history.”


Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice

Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 ± 3 °C colder than today.


Global carbon dioxide levels near worrisome milestone

Concentrations of greenhouse gas will soon surpass 400 parts per million at sentinel spot.

Near the moonscape summit of the Mauna Loa volcano in Hawaii, an infrared analyser will soon make history. Sometime in the next month, it is expected to record a daily concentration of carbon dioxide in the atmosphere of more than 400 parts per million (p.p.m.), a value not reached at this key surveillance point for a few million years.

There will be no balloons or noisemakers to celebrate the event. Researchers who monitor greenhouse gases will regard it more as a disturbing marker of humanity’s power to alter the chemistry of the atmosphere and by extension, the climate of the planet. At 400 p.p.m., nations will have a difficult time keeping global warming in check, says Corinne Le Quéré, a climate researcher at the University of East Anglia in Norwich, UK, who says that the impact “is getting very dangerously close to reaching the 2 °C target that governments around the world have pledged not to exceed”.

It will be a while, perhaps a few years, before the global CO2 concentration averaged over an entire year, passes 400 p.p.m.. But topping that value at Mauna Loa is significant because researchers have been monitoring the gas there since 1958, longer than any other spot. “It’s a time to take stock of where we are and where we’re going,” says Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, California, who oversees that centre’s CO2 monitoring efforts on Mauna Loa. That gas record, known as the Keeling curve, was started by his father, Charles Keeling.

When monitoring started, the CO2 level stood at 316 p.p.m., not much higher than the 280 p.p.m. that characterized conditions before the industrial revolution. But since the Hawaiian measurements began, the values have followed an upward slope that shows no sign of levelling off (see ‘On the rise’). Emissions of other greenhouse gases are also increasing, pushing the total equivalent concentration of CO2 in the atmosphere to around 478 p.p.m. in April, according to Ronald Prinn, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge.

Data compiled by Le Quéré and other members of the Global Carbon Project suggest that humans contributed around 10.4 billion tonnes of carbon into the atmosphere in 2011. About half of that is taken up each year by carbon ‘sinks’ such as the ocean and vegetation on land the rest remains in the atmosphere and raises the global concentration of CO2.

“The real question now is: how will the sinks behave in the future?” says Gregg Marland, an environmental scientist at Appalachian State University in Boone, North Carolina, who helps to compile the emissions data.

The sinks have grown substantially since Keeling began his measurements, when carbon emissions totalled about 2.5 billion tonnes a year. But climate models suggest that the land and ocean will not keep pace for long.

“At some point the planet can’t keep doing us a favour, particularly the terrestrial biosphere,” says Jim White, a biogeochemist at the University of Colorado Boulder. As the sinks slow down and more emitted CO2 stays in the atmosphere, levels will rise even faster.

Some researchers have suggested that the sinks have already started to clog up, reducing their ability to take up more CO2 (J. G. Canadell et al. Proc. Natl Acad. Sci. USA 104, 18866–18870 2007). Others disagree.

Ashley Ballantyne, a biogeochemist at the University of Montana in Missoula, worked with White and others to examine records of emissions as well as CO2 measurements made around the globe. They found no signs of sinks slowing down (A. P. Ballantyne et al. Nature 488, 70–72 2012). But it is difficult to be sure, says Inez Fung, a climate modeller at the University of California, Berkeley. “We don’t have adequate observing networks.” The largest global network, operated by the US National Oceanic and Atmospheric Administration, had to trim 12 stations in 2012 because of budget cuts.

Some of the most crucial areas, such as the tropics, are also the least monitored, although researchers are seeking to fill in the gaps. Scientists from Germany and Brazil are building a 300-metre tower to keep tabs on the Amazon (see Nature 467, 386–387 2010). And Europe’s Integrated Carbon Observation System is setting up stations throughout the continent and at some marine sites to measure CO2 and other greenhouse gases.

Satellites, too, could monitor carbon sources and sinks. Two orbiters are already providing some data, and NASA plans to launch the much anticipated Orbiting Carbon Observatory-2 next year (see page 5). An earlier version of that satellite failed during its 2009 launch.

Even as new resources come online, however, researchers are struggling to keep the Mauna Loa station going. “The amount of money that I’m able to obtain for the programme has diminished over time,” says Keeling, whose group monitors CO2 concentration at 13 sites around the world.

“It’s kind of silly that we chose to go all ostrich-like,” says White of the funding difficulties. “We don’t want to know how much CO2 is in the atmosphere, when we ought to be monitoring even more.”


Abstract

Permafrost and methane hydrates are large, climate-sensitive old carbon reservoirs that have the potential to emit large quantities of methane, a potent greenhouse gas, as the Earth continues to warm. We present ice core isotopic measurements of methane (Δ 14 C, δ 13 C, and δD) from the last deglaciation, which is a partial analog for modern warming. Our results show that methane emissions from old carbon reservoirs in response to deglacial warming were small (<19 teragrams of methane per year, 95% confidence interval) and argue against similar methane emissions in response to future warming. Our results also indicate that methane emissions from biomass burning in the pre-Industrial Holocene were 22 to 56 teragrams of methane per year (95% confidence interval), which is comparable to today.

Methane (CH4) is an important contributor to the greenhouse effect, with a global warming potential

28 times higher than that of carbon dioxide (CO2) on a 100-year time scale (1). Natural CH4 emissions currently account for

40% of total emissions (2) and there are considerable uncertainties in their response to future warming (3). Although wetlands are the dominant natural source of CH4, increased emissions from large, climate-sensitive old carbon reservoirs such as permafrost (4) and hydrates under ice sheets (5) might become important in the coming century. Marine hydrates may also have the potential to emit a substantial amount of CH4 into the atmosphere in response to warming (6), but the time scale of marine hydrate dissociation is relatively long (on the order of hundreds to thousands of years). Furthermore, there is a growing consensus that CH4 release to the atmosphere from dissociating marine hydrates will be buffered by efficient CH4 oxidation in the sediments and water column (3, 7).

The last deglaciation [18 to 8 kilo-annum before present (ka BP)] provides the opportunity for evaluating the long-term sensitivity of these old carbon reservoirs (marine hydrates, permafrost, and hydrates under ice sheets) to a changing climate. There is abundant evidence of the destabilization of marine hydrates (8, 9), land permafrost degradation (10), and thermokarst lake (permafrost thaw lake) formation (11) during the last deglaciation. However, CH4 emissions from these old carbon reservoirs into the atmosphere are not well constrained. The paleoatmospheric CH4 mole fraction and its isotopic composition from trapped air in ice cores provide a historical perspective on how natural CH4 sources respond to climate change (e.g., 12, 13). Measurements of carbon-14 ( 14 C) of CH4 ( 14 CH4) from ice cores specifically provide an unambiguous top-down constraint on the globally integrated 14 C-free CH4 emissions from all old carbon reservoirs.

14 C decays radioactively and is thus strongly depleted in carbon reservoirs that have been isolated from the atmosphere for time periods longer than its half-life of

5730 years. Because of the low abundance of 14 C (on the order of 10 −12 compared with 12 C), measurements of 14 CH4 in ice cores are challenging, requiring

1000 kg of ice per sample. We collected ice cores from a well-dated ice ablation site on Taylor Glacier, Antarctica (14), which provides easy access to large volumes of old ice at shallow depths. Petrenko et al. (15) recently presented measurements of paleoatmospheric 14 CH4 from Taylor Glacier for the Younger Dryas–Preboreal (YD-PB) transition (11.7 to 11.3 ka BP) and concluded that 14 C-free CH4 emissions were small [<7.7% of total CH4 emissions, 95% confidence interval (CI)]. However, their results only spanned a brief time interval within the deglacial transition. In this study, we present 11 additional measurements of paleoatmospheric 14 CH4 (Fig. 1A) combined with stable isotope measurements (δ 13 CH4 and δD-CH4) (Fig. 1, C and D) in the 15- to 8-ka BP time interval, providing a more complete picture of the deglacial CH4 budget.

(A) ∆ 14 CH4 from Taylor Glacier (blue diamonds this study), ∆ 14 C of contemporaneous CO2 from IntCal13 [green line (19)], IntCal13 raw data [gray crosses (19)], and earlier ∆ 14 CH4 results [light blue diamonds (15)]. Two ∆ 14 CH4 samples from the 2014–2015 field season (at 17.8 and 12.8 ka BP) were rejected because of suspected addition of extraneous 14 C [see section 3 of the materials and methods (20)]. (B) CH4 mole fraction from discrete WAIS Divide ice core measurements [red dots (39)], Taylor Glacier (blue diamonds this study), and an earlier Taylor Glacier study [light blue diamonds (15)]. (C) δ 13 CH4 from TALDICE (red squares), EDML [yellow squares (13)], and Taylor Glacier (blue squares this study). (D) δD-CH4 from EDML [green triangles (13)] and Taylor Glacier (blue triangles this study). (E) Composite NH temperature stack (red line) and its 95% CI (shaded orange area) (16). (F) Global RSL inferred from coral data (32). All ice core data are plotted with respect to the WD2014 age scale (40) IntCal13, RSL, and NH temperature stacks are plotted on their respective age scales. All error bars represent the 95% CI.

The Oldest Dryas–Bølling (OD-B) transition (14.6 to 14.45 ka BP) represents the first large and abrupt CH4 rise during the last deglacial sequence of events (Fig. 1B) at the time when sea level was

100 m lower than today. This abrupt CH4 rise was synchronous with the acceleration of Northern Hemisphere (NH) warming (16) (Fig. 1E), ice sheet retreat, and rapid sea-level rise (17). This climate transition may have also coincided with the first instance of marine hydrate destabilization during the last deglaciation caused by hydrostatic pressure relief from NH ice sheet retreat and incursion of warm intermediate ocean water into shallow, hydrate-bearing Arctic sediments (8). During the destabilization of marine hydrate reservoirs, abrupt events such as submarine landslides (18) or collapse of marine hydrate pingos (8) could result in large and rapid CH4 expulsions that may have contributed to the rapid atmospheric CH4 rise (9) if they were capable of bypassing oxidation in the water column.

In contrast to old carbon reservoirs, contemporaneous CH4 sources such as wetlands and biomass burning emit CH4 with a 14 C signature that reflects the contemporaneous Δ 14 CO2 at the time (15). Our Δ 14 CH4 measurements for the OD-B transition are all within 1σ uncertainty of the contemporaneous atmospheric Δ 14 CO2 (19) (Fig. 1A), indicating a dominant role of contemporaneous CH4 sources. We used a one-box model (see section 4.2 of the materials and methods) (20) to calculate the amount of 14 C-free CH4 emission into the atmosphere (Table 1, fig. S9, and table S10) (20). Our box model shows that the total 14 C-free CH4 emissions during the OD-B transition were small [on average, <13 teragrams (Tg) of CH4 per year, 95% CI upper limit]. Combined with earlier Δ 14 CH4 data from the YD-PB transition (15), our results argue strongly against the hypothesis regarding old carbon reservoirs being important contributors to the rapid CH4 increases associated with abrupt warming events (Dansgaard–Oeschger events) (9). This conclusion is consistent with previous studies (13) showing no major enrichment in the CH4 deuterium/hydrogen ratio (δD-CH4) concurrent with the abrupt CH4 transitions (CH4 from marine hydrates is relatively enriched in δD). It has been shown that even at a relatively shallow water depth of

90% of the 14 C-free CH4 released from thawing subsea permafrost was oxidized in the water column (21). We hypothesize that during the OD-B transition, relatively rapid sea-level rise associated with meltwater pulse 1-A (17), combined with CH4 oxidation in the water column (22), may have prevented CH4 emissions from disintegrating marine hydrates and sub-sea permafrost from reaching the atmosphere.

Sample ages were determined by value matching of globally well-mixed gases (CH4 and δ 18 O of atmospheric oxygen) to WD2014 chronology [see section 1 of the materials and methods (20)]. The sample ages given in this table represent the “best” (maximum probability) age on the probability distribution (fig. S3) (20) with respect to WD2014 chronology (40).

Our measurements of 14 CH4 during the Bølling–Allerød interstadial (14.45 to 13 ka BP) and the early Holocene (10 to 8 ka BP) warm period (Fig. 1A) provide an opportunity to assess the likelihood of delayed CH4 emissions from old carbon reservoirs in response to warming. The onset of marine hydrate dissociation might lag the initial warming signal on decadal (23), centennial, or even millennial (18) time scales. Permafrost degradation could also lag a warming signal on decadal and centennial time scales (24) depending on local environmental conditions such as permafrost depth, soil types, and moisture content (4). During parts of the early Holocene, Arctic temperatures were likely warmer than today (25), providing a good analog for Arctic conditions in the coming decades. Proxy reconstructions of thermokarst lake initiation (11) and land permafrost degradation (10, 24) suggested a potential increase of CH4 emissions from these processes during both the Bølling–Allerød interstadial and the early Holocene warm period. However, our Δ 14 CH4 measurements (Fig. 1A and Table 1) show no evidence of delayed 14 C-free CH4 emissions after warming. These results are consistent with present-day observations that carbon from thermokarst lakes and permafrost is predominantly emitted in the form of CO2 rather than CH4 (4, 26), and that CH4 emissions from permafrost systems are dominated by relatively contemporaneous carbon (26, 27).

Because carbon stored in permafrost is not expected to be 14 C free (28), we also attempted to use our 14 CH4 results to calculate the possible magnitude of CH4 emissions from thawing old carbon in permafrost (Section 4.3) (20). This calculation assumed that the 14 C activity of permafrost CH4 emissions follows the predepositional age of terrigenous biomarkers released from thawing permafrost (7500 ± 2500 years old relative to our sample age) (10). Resulting CH4 emissions from old permafrost carbon range from 0 to 53 Tg CH4 per year (table S10) (20) throughout the last deglaciation and may have contributed up to 27% of the total CH4 emissions to the atmosphere (95% CI upper limit) at the end of the OD-B transition (14.42 ka BP). However, we consider this calculation speculative (see section 4.3 of the materials and methods) (20).

When the global sea level was lower, exposure of continental shelves may have resulted in higher CH4 emissions from natural geologic seeps (29). A recent study also inferred the existence of CH4 hydrate deposits underneath ice sheets and suggested that the proglacial meltwater discharge is likely an important source of CH4 to the atmosphere (5). Ice sheet retreat during the last deglaciation may have destabilized the subglacial hydrate deposits, which contain old, 14 C-depleted CH4. However, our data, which span most of the deglacial ice retreat and sea-level rise (Fig. 1F), argue strongly against both hypotheses. The 14 C-free CH4 emissions were small throughout the last deglaciation (Table 1) and appear to be insensitive to both global sea level and ice volume.

Biomass burning is an important component of the global carbon cycle and is tightly coupled with emissions of carbon monoxide (CO), nitrogen oxides (NOx), nonmethane hydrocarbons, and aerosols that have substantial effects on atmospheric chemistry and radiative energy fluxes. Compared with other proxies of past biomass burning, CH4 has an advantage because it is a well-mixed gas in the atmosphere and can represent the globally integrated biomass-burning emissions. Bock et al. (13) provided the most recent stable isotope–based (δ 13 C and δD) study of the glacial–interglacial CH4 budget, but they were unable to separate the relative contributions from CH4 sources that are enriched in heavier isotopes (biomass burning and natural geologic emissions). With improved estimates of natural geologic emissions, our results allow for better constraints on the overall CH4 budget. We used the stable isotope data (Fig. 1, C and D) in a one-box model (see section 5 of the materials and methods) (20) to calculate CH4 emissions from biomass burning (CH4 bb) and microbial sources (CH4 mic, composed of emissions from wetlands, ruminants, and termites) for the Early Holocene (Table 1 and fig. S11) (20). We extended our calculation to the late Holocene (

2 ka BP) (Table 1) to directly compare our CH4 source strength estimates with those of earlier studies (30, 31). This assumption can be justified because a large change in the natural geologic emissions between the early Holocene and 2 ka BP seems unlikely because global sea level and ice volume did not change appreciably after 8 ka BP (32). However, we did not perform this calculation for the pre-Holocene samples because estimates of the CH4 interpolar difference, atmospheric global average CH4 stable isotope values, and stable isotopic signatures of the sources are more uncertain (Section 5) (20).

We calculated relatively high CH4 bb emissions in the early Holocene (33 to 56 Tg CH4 per year, 95% CI) at 10 ka BP and a slight decrease of CH4 bb emissions (22 to 42 Tg CH4 per year, 95% CI) toward the late Holocene (Table 1). However, the magnitude of the decrease in biomass-burning emissions (

7 Tg CH4 per year) is small relative to the uncertainties for both the CH4 bb and CH4 mic emissions (±11 and ±18 Tg CH4 per year, respectively, 95% CI uncertainties). Our estimate of 22 to 42 Tg CH4 per year (95% CI) CH4 bb emissions for the late Holocene period (

2 ka BP) is within the upper range of estimates from previous ice core studies (13, 30, 31). Considering the large downward revision of natural geologic emissions inferred from our 14 C data, an upward revision in pyrogenic CH4 emissions is expected to balance the CH4 stable isotope budget. The increase in CH4 bb expected from a reduction in natural geologic emissions is partly offset by a –0.5 to –1‰ revision in atmospheric δ 13 CH4 values (12, 30, 31) because the δ 13 CH4 values from earlier studies (30, 31) were likely biased because of krypton (Kr) interference (33). Our CH4 bb estimates are also reduced because, unlike previous studies, we accounted for temporal shifts in the isotopic signatures of CH4 bb and CH4 mic between the pre-Industrial Holocene and the modern period expected from anthropogenically driven changes in the δ 13 CO2 precursor material and land use (see section 5.2 of the materials and methods) (20). Our best CH4 bb estimates for the late Holocene (22 to 42 Tg CH4 per year, 95% CI) are comparable to the present-day estimates of combined pyrogenic CH4 emissions from anthropogenic biomass burning and wildfires (2). This result is supported by some (34, 35), but not all (36), independent paleoproxies of biomass burning.

The last deglaciation serves only as a partial analog to current anthropogenic warming, with the most important differences being the much colder baseline temperature, lower sea level, and the presence of large ice sheets covering a large part of what are currently permafrost regions in the NH. Although Arctic temperatures during the peak early Holocene warmth were likely warmer than today (25), they were still lower than the Arctic temperature projections by the end of this century under most warming scenarios (37). However, there are also many similarities between the last deglaciation and current anthropogenic warming. Both deglacial and modern warming include strong Arctic amplification, and the magnitude of global warming (

4°C) (16) during the last deglaciation was comparable to the expected magnitude of equilibrium global temperature change under midrange anthropogenic emission scenarios (37). Because the relatively large global warming of the last deglaciation (which included periods of large and rapid regional warming in the high latitudes) did not trigger CH4 emissions from old carbon reservoirs, such CH4 emissions in response to anthropogenic warming also appear to be unlikely. Our results instead support the hypothesis that natural CH4 emissions involving contemporaneous carbon from wetlands are likely to increase as warming continues (38). We also estimated relatively high CH4 bb emissions for the pre-Industrial Holocene that were comparable to present-day combined pyrogenic CH4 emissions from natural and anthropogenic sources. This result suggests either an underestimation of present-day CH4 bb or a two-way anthropogenic influence on fire activity during the Industrial Revolution: reduction in wildfires from active fire suppression and landscape fragmentation balanced by increased fire emissions from land-use change (deforestation) and traditional biofuel use (burning of plant materials for cooking and heating).


Abstract

A newly developed isotope ratio laser spectrometer for CO2 analyses has been tested during a tracer experiment at the Ketzin pilot site (northern Germany) for CO2 storage. For the experiment, 500 tons of CO2 from a natural CO2 reservoir was injected in supercritical state into the reservoir. The carbon stable isotope value (δ 13 C) of injected CO2 was significantly different from background values. In order to observe the breakthrough of the isotope tracer continuously, the new instruments were connected to a stainless steel riser tube that was installed in an observation well. The laser instrument is based on tunable laser direct absorption in the mid-infrared. The instrument recorded a continuous 10 day carbon stable isotope data set with 30 min resolution directly on-site in a field-based laboratory container during a tracer experiment. To test the instruments performance and accuracy the monitoring campaign was accompanied by daily CO2 sampling for laboratory analyses with isotope ratio mass spectrometry (IRMS). The carbon stable isotope ratios measured by conventional IRMS technique and by the new mid-infrared laser spectrometer agree remarkably well within analytical precision. This proves the capability of the new mid-infrared direct absorption technique to measure high precision and accurate real-time stable isotope data directly in the field. The laser spectroscopy data revealed for the first time a prior to this experiment unknown, intensive dynamic with fast changing δ 13 C values. The arrival pattern of the tracer suggest that the observed fluctuations were probably caused by migration along separate and distinct preferential flow paths between injection well and observation well. The short-term variances as observed in this study might have been missed during previous works that applied laboratory-based IRMS analysis. The new technique could contribute to a better tracing of the migration of the underground CO2 plume and help to ensure the long-term integrity of the reservoir.


Introduction

Forests, Carbon, and the Additional Benefits of Woodlands

Global forests store about a trillion tons of carbon [1]. Forests—whether temperate or tropical, and with closed or open canopy—are the largest terrestrial sink of carbon, comprising about 25% of the planetary carbon budget [2]. This is roughly equivalent to the carbon sequestered, or kept out of the atmosphere, by the oceans [3]. The 2015 Paris Climate Agreement among 196 countries calls for achieving a balance between the anthropogenic emissions by sources and removal by sinks in the second half of this century. Most temperate zone and developed world strategies focus on cutting carbon emissions through changes in technology and energy consumption in order to “bend the curve” of climate change below the projected 2+ degrees centigrade. However, to achieve the Paris goals, enhancement of forest-based carbon (C) removals to mitigate emissions in other sectors will be a critical component of any collective global strategy for achieving carbon neutrality [4,5]. Any attempt at carbon neutrality must have significant forest and landscape dimensions. Forests cover a large area of the planet, especially in comparison to the 3% of the Earth’s surface occupied by cities. In the short term, carbon uptake by vegetation and storage in biotic systems is one of the most rapid and promising strategies for addressing emissions.

In the United States (US), Carbon sequestration in forests offsets about 10–15% of emissions from transportation and energy sources and may help to significantly reduce the overall costs of achieving emission targets set by the Paris Agreement [1]. Without improving the extent, health, and productivity of these forests, the sequestration capacity may reduce because of climate change and increasing disturbance [6]. Many climate change adaptation enterprises will certainly involve enhancing tree landscapes at many scales. Such improvements provide additional “ecosystem services,” or positive impacts for people, from shading buildings and buffering cities against storms to making agricultural and grazing landscapes more productive.

With the recent prominence of Reduced Emissions from Deforestation and Degradation (REDD+), more than sixty, mostly tropical, countries place forests at the center of their climate strategies as part of the 2015 Paris Climate Accords, which make special provision “to conserve and enhance sinks and reservoirs of greenhouse gases through results based payments”—which is more generally known as REDD+. While many discussions of climate solutions focus on technological change, energy demand, and reactivating energy resources such as nuclear power, there are significant and rapid carbon uptake gains to be made through managing landscape systems. Changes in landscape management are generally more decentralized than changes in technology and energy, especially in the tropics where most of this sequestration and storage takes place [7,8,9]. We also emphasize that there are gains to be made “at the margins” through improvement of secondary, agricultural, and urban forests with positive mitigation and adaptation outcomes.

Many technological solutions to climate change define the benefits by human gains and goals. These approaches usually require rarified knowledge systems and complex technologies such as electric cars and solar panels they have narrowly specified outcomes and are often highly monetized. In contrast, forest and landscape improvement provides many additional benefits for humans, non-humans, and biophysical processes with relatively low entry and management costs. These co-benefits—or environmental services—improve the health of the biosphere as well as the hydrological and microclimatic systems that play an important role in the maintaining the carbon sequestration capacity of the Earth. This “broad spectrum” quite direct enhancement, in addition to GHG uptake and storage, is unmatched by any other intervention to avoid climate disruption.

We frame this paper by exploring the multifunctionality of arboreal systems, including their carbon uptake (or sequestration) and storage. We emphasize the importance not only of dense tropical forests, but also of inhabited landscapes shaped by people—such as secondary forests, mixed agricultural systems, and cities and their environs—and discuss where such landscapes fit in climate policy and practices. We begin by introducing the ideas of multifunctionality and climate justice, but then move to specific contributions to carbon uptake in a range of forest types, including “agroforests,” or forests people use to grow food, as well as urban and peri-urban forests. We conclude with the question of GHG uptake in urban areas and how researchers are rethinking the greenhouse gas footprint of cities, including urban waste. We emphasize that “bending the curve” of climate change below 2+ degrees centigrade is not simply a technical issue of planting more trees, although that is part of it. “Bending the curve” also involves reassessing our relationship to nature and creating political economies, institutions, and practices that support biotic processes as one of the central responses to climate change.

Forest Multifunctionality

Woodlands ranging from the high biomass forests of the humid tropics to the peri-urban and urban arborizations, especially in the developing world, all provide ecosystem services that go well beyond carbon. Many of these are summarized in the Table 1.

The Multifunctionality and Co-Benefits of Woodlands.

1. Biodiversity benefits, including

ecological and habitat connectivity

ecological services such as pollination, commensal support, predation, seed distribution, and food supply.

2. Agricultural benefits, including

soil fertility improvements in some cases

3. Soil benefits, including

increasing organic matter in the soil and improving soil structure.

buffering the impacts of rainfall

transpiration (taking up moisture through the roots and releasing it through the leaves)

recharging the moisture in the soil

moderating the flow of streams

5. Microclimate improvements, especially for

moderating urban heat island effects [16,17,18]

reduction of heat stress in agroforestry and silvo-pastoral systems [16,19]

6. Local weather defense, including

shoreline protection via mangroves

7. Economic benefits, such as

producing timber and posts

producing non-timber products, such as resins, latexes, medicines, oil seeds, and stimulants like coffee and teas

producing commercial commodities, such as coffee, tea, cacao, and so on

potential REDD derivatives or other offset initiatives pertaining to carbon.

8. Subsistence benefits, such as

providing food to people who live in or near forests

providing fodder for livestock

providing construction materials

9. Survival benefits and complex livelihood “insurance,” such as

10. Human symbolic meaning, including

1. Biodiversity benefits, including

ecological and habitat connectivity

ecological services such as pollination, commensal support, predation, seed distribution, and food supply.

2. Agricultural benefits, including

soil fertility improvements in some cases

3. Soil benefits, including

increasing organic matter in the soil and improving soil structure.

buffering the impacts of rainfall

transpiration (taking up moisture through the roots and releasing it through the leaves)

recharging the moisture in the soil

moderating the flow of streams

5. Microclimate improvements, especially for

moderating urban heat island effects [16,17,18]

reduction of heat stress in agroforestry and silvo-pastoral systems [16,19]

6. Local weather defense, including

shoreline protection via mangroves

7. Economic benefits, such as

producing timber and posts

producing non-timber products, such as resins, latexes, medicines, oil seeds, and stimulants like coffee and teas

producing commercial commodities, such as coffee, tea, cacao, and so on

potential REDD derivatives or other offset initiatives pertaining to carbon.

8. Subsistence benefits, such as

providing food to people who live in or near forests

providing fodder for livestock

providing construction materials

9. Survival benefits and complex livelihood “insurance,” such as

10. Human symbolic meaning, including

This impressive list of additional benefits provided by tree systems helps explain why between 800,000 and 1.4 billion people on the planet are at least periodically dependent on forest resources for their livelihoods, labor markets, agricultural inputs, building and artisanal materials, subsistence, and survival “insurance” in difficult times [20,21,22,23,24,25,26]. North American mainstream views of the environment that strongly segment land uses have difficulty “seeing” such heterogeneous systems in part because of the conceptual construction (and constriction) of “types” of nature into wild, agricultural, and urban systems which are assumed to have little overlap. This perception is far less prevalent in the developing world, but these separations, which have a venerable history, have led to many policy distortions [27]. The fact that human use of woodlands can be periodic, seasonal, dispersed, or indirect further obscures the importance of forested landscapes.

Forests reflect biotic, social, and symbolic systems. Forests occur in wild landscapes, in inhabited and working landscapes of varying forms and intensities, and in highly “unnatural landscapes” like cities. The ubiquity and extent of forests also contributes to their invisibility. Woodlands are culturally complex they have rich social and ecological capacities as well as social and ecological vulnerabilities. Forests embody ideologies, knowledge regimes, institutional approaches to land control and land access, human symbolic meaning, sensitivity to economic signals, and diverse power relations among local, national, and international stakeholders. While woodlands and pastures are generally viewed as parts of wild or distant nature, in this chapter we emphasize the pervasive arboreal nature of even urban areas as critical sites of woody and other biota-based “carbon plus” environmental services. Just as an example, in a survey of over a thousand urban households in South Africa, non-timber forests products contributed 20% of household income [28,29,30], a finding hardly unique to South Africa [25,31,32,33]. Animal production is also often a considerable part of urban food production in cities, both in the developing world and the US [34,35,36].

Peri-urban areas—or areas surrounding cities—are also increasingly important in this regard as intersections between wildlands, agricultural lands, and cities. Peri-urban areas often host complex agronomic systems with tree components on the urban fringes, in landscapes through which people migrate to the city [35,37,38,39,40,41,42,43].

Far more than any other climate mitigation or adaptation “technology,” forest systems of multiple types engage large portions of the planet’s residents. People of many cultures, backgrounds, and material capacities are, in fact, already taking part in global woodland dynamics as part of formal and informal systems of management and access, as well as through consumption of forest products, economic activity, and aesthetic and symbolic practices. Landscape systems are by far the most inclusive forms of intervention for “bending the curve” of climate change below 2+ degrees centrigrade. This helps explain why wooded landscapes from wildlands to urban regions produce faster results for GHG uptake and at larger scales than most other technological interventions in carbon mitigation, as we will show later in this paper.

Our own Western enchantment with technology blinds us to the importance of living landscapes and the contributions of their “soft technologies.” In part, this is because the management and stewardship of woodlands is imbricated in a vast set of social relations, institutions, socio-political forces, economic imperatives, and global pressures that are not especially amenable to reductionist analysis, uniform scales, or even necessarily classic forms of scientific inquiry. Further, these systems are ubiquitous, although very under-appreciated, and for this reason, some of the urban and peri-urban dynamics of woodlands and their “footprints” remain almost invisible [23,42,44,45,46,47]. These kinds of “invisibilities” have occluded attention to secondary forests and extensive home gardens for decades [48].

Climate Justice

The term climate justice, when used in a restricted sense for policy purposes, means addressing the economic disparity between those societies that now generate and have historically generated most GHGs, on one hand, and those that have borne the brunt of the effects of climate change, on the other. Climate justice involves not only compensating those who suffer the consequences of climate instabilities [49,50], but also, some argue, allowing them to participate in developing policies with climate consequences that affect them (such as policies about mining, REDD, the siting of pipelines and processing plants, and so on). A definition of climate justice that goes beyond economics (including a normative call for intergenerational equity, resources transfers, and sustainable development) can be found in chapter 8 of this report.

The decentralized nature of the problem of climate justice, the question of intentionality, and the difficulty of taking collective action to address climate injustice present serious ethical and practical challenges. These challenges involve problems of scale, unforeseen impacts, interactive outcomes among agents, power relations, and diffuse consequences that dramatically transform the vulnerabilities of populations whose carbon footprint and historic responsibility for planetary carbon loads and other GHGs are minimal. These indirect effects are compounded by globally divergent consumption patterns, limited capacities for resilience of states and communities, and augmented vulnerabilities [51]. The current explosive fires in the American west, continuing “record” flooding in the Mississippi and Missouri valleys, and hyper severe tornedo seasons highlight that climate justice and climate vulnerability is a class issue in environmental justice in developed countries as well.

The means of compensation so far have mainly taken the form of fiscal transfers, provisioning of social services, and in some cases infrastructure improvement. Broader approaches could include support for rural livelihoods, improvement of urban and peri-urban biotic amenities, jobs, compensation for environmental services (such as but not limited to REDD), adaptation investments and programs that focus on reducing vulnerabilities of regions and populations most at risk from climate change. Economic support for carbon absorptive production systems like agroforestry, urban community arborization, conservation investments within inhabited landscapes, and new institutions and ideologies that support such approaches can enact a wide number of interventions, seeking input from local populations and capitalizing on local innovations [52,53,54,55].

REDD might usefully focus on secondary and agro-forests, but so far most carbon offsets have emphasized standing old growth forests with conservation support, such as Noel Kempff Mercado National Park in Bolivia and the Juma Reserve in Amazonas [53,55,56,57,58]. Brazil’s “Bolsa Florestal” program and Ecuador’s “Socio-Bosque” program provide a modest subsidy to forest dwellers to conserve forests and alleviate poverty. Such REDD+ programs have raised many questions about tenurial arrangements (who owns and who has rights to occupy and use the land and other resources), distribution of economic benefits, inclusion, competition among governance strategies and institutions, and compliance and monitoring. All of these questions have significant climate justice implications [58,59,60]. While many actors are trying to build flexibility into the programs, REDD runs the risk of being excessively overarching and falling prey to the vice of becoming a “development fad,” abandoned and reviled a few years later. Given the problems that currently plague the carbon cap and trade markets, this is a real risk for REDD programs specifically and to addressing problems at the “transnational level” in general. Global policies may be unable to deal with resistance on the ground in part, this results from the importance of forest goods in people’s livelihoods and to their wellbeing. Article Five of the Paris Accords helped draw global attention to forests, but most of the language revolves around “wildlands,” rather than working landscapes, and many complexities remain [58,61,62]. Such working woodland areas are crucial for livelihoods and livelihood supplements in rural and urban economies throughout the world, where an estimated billion people are forest-dependent to some degree [33,63,64,65,66]. In a recent transnational set of studies in rural areas, about 30% of the livelihood products—including food, forage, fuel, building materials, and so on—were derived from forest ecosystems [67,68,69,70].

Smaller Scale, Bigger Impact?

Many subnational approaches, such as the 100 Resilient Cities initiative, seem to have more traction on climate justice concerns. As international REDD programs wait to get off the ground, national governments increasingly look to regional forests to offset their own emissions. This actually puts forest questions at the heart of climate justice issues, since most rural development policy increasingly focuses on a few global and regional markets and high-input commodities. While forest policy has garnered increased visibility, attention to it has revolved strongly around conservation and climate. Development policies focused on forest-based rural livelihoods have received less attention, in spite of the best efforts of international organizations such as the Center for International Forestry Research (CIFOR) and La Via Campesina, the international peasant movement for small-scale sustainable agriculture [71,72,73,74].

Access to forests and their products are changing, and traditional uses may be criminalized in some GHG offset regimes [9,53,75,76,77]. Insecure tenure regimes may precipitate land grabs and forest conversion. For this reason, it is essential to work with local communities and with multiple forms of local knowledge in order to design effective systems. We must make sure that carbon offsets do not become a new form of expropriation, assuaging the guilt of GHG gluttons while marginalizing and criminalizing those whose livelihoods depend on functioning forests. This is a critique that is regularly leveled at REDD. Woody systems have the potential to both sequester carbon and help alleviate poverty through subsistence and market goods, although the magnitude remains controversial [78,79,80,81,82].

In the next sections we outline several dynamics that we suggest have important effects for bending the curve. We look at six processes in terms of both how they can mitigate climate change and how they can help people and ecosystems adapt to it. These processes are: 1) slowing deforestation 2) forest resurgence 3) agroforestry and matrix systems 4) urban and peri-urban forests in carbon dynamics and finally 5) the urban waste system and methane management. All these strategies occur within highly conjunctural social, market, institutional, cultural, and environmental conditions of possibility, and all are highly interactive and reactive to economic, environmental, and political volatilities. History, economics, politics, culture, institutions, and questions of epistemology shape these dynamics far more than we imagine.


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The moving of high emission for biomass burning in China: View from multi-year emission estimation and human-driven forces

Biomass burning (BB) has significant impacts on air quality, climate and human health. In China, the BB emission has changed substantially over the past decades while the multi-year variation held high uncertainty and the driving forces have addressed little attention. Here, this research aimed to conduct a comprehensive and systematic analysis of BB variation in China and provided precise and targeted BB emission reduction suggestions. The moving of high emission for BB from 2003 to 2014 was clearly identified, by the view of reliable emission estimation and anthropogenic impacts. Multiple satellite products, field survey, time varying biomass loading data and measured emission factors were adopted to better estimating BB emission and reducing the uncertainty. Social-economic analysis was added to assess the anthropogenic impacts on high emission variation quantitatively. Results showed that the cumulative BB emissions of OC, EC, CH4, NOX, NMVOC, SO2, NH3, CO, CO2, PM2.5 and PM10 during 2003-2014 were 1.6 × 10 4 , 5.64 × 10 3 , 3.57 × 10 4 , 1.7 × 10 4 , 5.44 × 10 4 , 2.96 × 10 3 , 6.77 × 10 3 , 6.5 × 10 5 , 1.15 × 10 7 , 5.26 × 10 4 and 6.04 × 10 4 Gg, respectively. Crop straw burning (in-field and domestic) in northeast China plain (NEP), north China plain (NCP), northern arid and semiarid region and loess plateau were the key sources, averagely contributed 73% for all the pollutants emission. While domestic straw burning and firewood burning in Sichuan basin (SB), Yunnan-Guizhou plateau and southern China were main contributors, averagely accounting for 70% of all the pollutants emission. On regional level, high emissions were mainly found in SB, NCP and NEP. Temporally, high emissions were mainly found in crop sowing harvesting and heating seasons. From 2003 to 2014, the BB emission for different biomass species has changed significantly in different regions. High emission has gradually moved from SB to NCP and NEP. Firewood burning and domestic straw burning emission decreased by 47% and 14% in SB, respectively. In-field straw burning emission increased by 52% and 231% in NCP and NEP respectively and domestic straw burning emission increased by 62% in NEP. Emissions from heating season have decreased while emissions in corn harvest season were continuously increased. Analysis of Environmental kuznets curve, agricultural productivity level, human burning habits, rural energy structure and local control policies revealed the internal human driving strength of the variation for BB emission. The unbalanced development of social economy and the policy bias were primary drivers of limiting the BB management. BB emission will alleviate in NCP and aggravate in NEP. For the further emission reduction, effective measures for corn sources management, straw returning and rural energy utilization should be systematically considered. This research provides a clear evidence for the multi-year variation pattern of BB emissions, which is critical for pollution prediction, air quality modeling and targeted mitigation strategies for the key regions of China.

Keywords: Biomass burning High emission variation Human-driven forces Multiple satellite data Social-economic analysis.

Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.


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