Greenhouse Gases and the Carbon Cycle
- Global carbon dioxide (CO2) emissions from fossil fuel burning in 2008 were 40% higher than those in 1990, with a three-fold acceleration over the past 18 years.
- Global CO2 emissions from fossil fuel burning are tracking near the highest scenarios considered so far by the IPCC.
The Atmosphere
- Global air temperature, humidity and rainfall trend patterns exhibit a distinct fingerprint that cannot be explained by phenomena apart from increased atmospheric greenhouse gas concentrations.
- Every year this century (2001-2008) has been among the top 10 warmest years since instrumental records began, despite solar irradiance being relatively weak over the past few years.
- Global atmospheric temperatures maintain a strong warming trend since the 1970s (~0.6°C), consistent with expectations of greenhouse induced warming.
Extreme Events
The IPCC Fourth Assessment Report concluded that many changes in extremes had been observed since the 1970s …
- more frequent hot days, hot nights and heat waves;
- fewer cold days, cold nights and frosts;
- more frequent heavy precipitation events;
- more intense and longer droughts over wider areas; and
- an increase in intense tropical cyclone activity in the North Atlantic but no trend in total numbers of tropical cyclones.
Land Surface
- Land cover change, particularly deforestation, can have a major impact on regional climate, but at the global scale its biggest impact comes from the CO2 released in the process. …
- Carbon dioxide changes during the Little Ice Age indicate that warming may in turn lead to carbon release from land surfaces, a feedback that could amplify 21st century climate change.
Permafrost and Hydrates
- New insights into the Northern Hemisphere permafrost (permanently frozen ground) suggest a large potential source of CO2 and CH4 that would amplify atmospheric concentrations if released.
- A recent increase in global methane levels cannot yet be attributed to permafrost degradation.
- A separate and significant source of methane exists as hydrates beneath the deep ocean floor and in permafrost. …
Contents
Greenhouse Gases and the Carbon Cycle
The Atmosphere
Extreme Events
Land Surface
Permafrost and Hydrates
Climate Change Research Centre
- The Copenhagen Diagnosis, 2009: Updating the World on the Latest Climate Science, Climate Change Research Centre, The University of New South Wales, Sydney, 2009.
Ian Allison, Nathan Bindoff, Robert Bindschadler, Peter Cox,Nathalie de Noblet-Ducoudré, Matthew England, Jane Francis, Nicolas Gruber, Alan Haywood, David Karoly, Georg Kaser, Corinne Le Quéré, Tim Lenton, Michael Mann, Ben McNeil, Andy Pitman, Stefan Rahmstorf, Eric Rignot, Hans Joachim Schellnhuber, Stephen Schneider, Steven Sherwood, Richard Somerville, Konrad Steffen, Eric Steig, Martin Visbeck and Andrew Weaver.
Greenhouse Gases and the Carbon Cycle
Global Carbon Dioxide Emissions
The accelerated growth in fossil fuel CO2 emissions since 2000 was primarily caused by fast growth rates in developing countries (particularly China) in part due to increased international trade of goods, and by the slowdown of previous improvements in the CO2 intensity of the global economy. …
Preliminary figures suggest total CO2 emissions have dropped in 2009, but this is a temporary effect resulting from the global recession …
Figure 1
Emissions in 2009 are projected to be ~3% below 2008 levels, close to the level of emissions in 2007.
This reduction is equivalent to a temporary halt in global emissions for a period of only 2-4 weeks.
Carbon Dioxide
The concentration of CO2 in the atmosphere reached 385 parts per million (ppm) in 2008.
The atmospheric CO2 concentration is more than 105 ppm above its natural preindustrial level.
The present concentration is higher than at any time in the last 800,000 years, and potentially the last 3 to 20 million years. …
This rate of increase of atmospheric CO2 is more than ten times faster than the highest rate that has been detected in ice core data [ie] at any time in the last 22,000 years.
Methane
The concentration of methane (CH4) in the atmosphere increased since 2007 to 1800 parts per billion (ppb) after almost a decade of little change. …
The spatial distribution of the CH4 increase shows that an increase in Northern Hemisphere CH4 emissions has played a role and could dominate the signal, but the source of the increase is unknown.
(p 9)
Annual industrial emissions of CH4 are not available as they are difficult to quantify. …
Carbon Sinks and Future Vulnerabilities
The oceanic and terrestrial CO2 reservoirs — the ‘CO2 sinks’— have continued to absorb more than half of the total emissions of CO2.
However the fraction of emissions absorbed by the reservoirs has likely decreased by ~5% (from 60 to 55%) in the past 50 years.
The uncertainty in this estimate is large [due to] interannual variability and … land use change. …
Our current understanding indicates that the natural CO2 sinks will decrease in efficiency during this century, and the terrestrial sink could even start to emit CO2.
The response of the sinks to elevated CO2 and climate change is shown in models to amplify global warming by 5-30%.
Is the greenhouse effect already saturated, so that adding more CO2 makes no difference?
(p 10)
No, not even remotely.
It isn’t even saturated on the runaway greenhouse planet Venus, with its atmosphere made up of 96% CO2 and a surface temperature of 467°C, hotter even than Mercury.
The Atmosphere
Global Temperature Trends
The atmospheric warming trend continues to climb despite 2008 being cooler than 2007.
[The] IPCC gave the 25-year trend as 0.177 ± 0.052 °C per decade for the period ending 2006 (based on the HadCRUT data).
Updating this by including the last two years (2007 and 2008), the trend becomes 0.187 ± 0.052 °C per decade for the period ending 2008.
[In] 2008 a La Niña occurred, a climate pattern which naturally causes a temporary dip in the average global temperature.
At the same time, solar output was also at its lowest level of the satellite era …
(p 11)
Is the Warming Natural or Human-Induced?
By far the greatest part of the observed century-scale warming is due to human factors. …
[The] sun contributed only about 10% of surface warming in the last century and a negligible amount in the last quarter century …
No credible scientific literature has been published since the AR4 assessment that supports alternative hypotheses to explain the warming trend.
Is Warming Occurring Higher up in the Atmosphere?
Most data sets available at that time [of the AR4] showed weaker than expected warming in the atmospheric region referred to as the tropical upper troposphere, ten to fifteen kilometers above the surface. …
Researchers have since performed additional analyses of the same data using more rigorous techniques, and developed a new method of assessing temperature trends from wind observations.
The new observational estimates show greater warming than the earlier ones [resolving] a significant ambiguity expressed in AR4.
Water Vapor, Rainfall and the Hydrological Cycle
New research and observations have [confirmed that] a warming climate will lead to an atmosphere containing more water vapor, which would add to the greenhouse effect and enhance the warming. …
Satellite data show that atmospheric moisture content over the oceans has increased since 1998 …
[R]ainfall has reduced in the Northern Hemisphere subtropics but has increased in middle latitudes …
(p 12)
Has global warming recently slowed down or paused?
(p 13)
There is no indication in the data of a slow down or pause in the human-caused climatic warming trend. …
[NeitherEl Niño, nor solar activity or volcanic eruptions make a significant contribution to longer-term climate trends. …
[Global] cooling has not occurred even over the past ten years …
The Hadley Center data [shows] smaller warming trends [primarily because it] leaves out the Arctic, which has warmed particularly strongly in recent years.
[Despite] the extremely low brightness of the sun over the past three years [—] temperature records have been broken during this time.
For example,- March 2008 saw the warmest global land temperature of any March ever measured in the instrumental record.
- June and August 2009 saw the warmest land and ocean temperatures in the Southern Hemisphere ever recorded for those months. …
- The years 2007, 2008 and 2009 had the lowest summer Arctic sea ice cover ever recorded, and in 2008 for the first time in living memory the Northwest Passage and the Northeast Passage were simultaneously ice-free.
This feat was repeated in 2009. - Every single year of this century (2001-2008) has been among the top ten warmest years since instrumental records began.
Can solar activity of other natural processes explain global warming?
[Incoming] solar radiation has been almost constant over the past 50 years, apart from the well-known 11-year solar cycle [and] has slightly decreased over this period.
[Over] the past three years the brightness of the sun has reached an all-time low since the beginning of satellite measurements in the 1970s …
[This] natural cooling effect was more than a factor of ten smaller than the effect of increasing greenhouse gases …
[Winters] are warming more rapidly than summers, and overnight minimum temperatures have warmed more rapidly than the daytime maxima — exactly the opposite of what would be the case if the sun were causing the warming.
Figure 5
(p 14)
Time-series of solar irradiance alongside the net effect of greenhouse gas emissions … calculated in terms of total estimated impact on global air temperatures:- observed from 1970-2008; and
- projected from 2009-2030
Extreme Events
Temperature extremes
Continued marked increases in hot extremes and decreases in cold extremes are expected in most areas across the globe due to further anthropogenic climate change.
Precipitation extremes and drought
[Rainfall has] become more intense in already-rainy areas as atmospheric water vapor content increases.
[Recent] changes have occurred even faster than predicted, raising the possibility that future changes could be more severe than predicted.
[Recent] increases in heavy precipitation is found in the United States … are consistent with the warming of the climate [system; however,] it has not been possible to attribute them to anthropogenic climate change with high confidence due to the very large variability of precipitation extremes.
(p 15)
[There] have also been observed increases in drought since the 1970s, consistent with the decreases in mean precipitation over land in some latitude bands that have been attributed to anthropogenic climate change.
[Current] studies suggest that heavy precipitation rates may increase by 5% - 10% per °C of warming, similar to the rate of increase of atmospheric water vapor. …
Tropical cyclones
[Studies] since the IPCC report have … found a global increase in the number of hurricanes of the strongest categories 4 and 5, and … identified rising sea surface temperatures (SST) as the leading cause.
A complete reanalysis of satellite data since 1980 … found a 1°C global warming corresponding to a 30% increase in these storms.
[However, there is] as yet no robust capacity to project future changes in tropical cyclone activity.
Other severe weather events
[Recent] research has shown an increased frequency of severe thunderstorms in some regions, particularly the tropics and south-eastern US, is expected due to future anthropogenic climate change.
[There] have been recent increases in the frequency and intensity of wildfires in many regions with Mediterranean climates (eg Spain, Greece, southern California, south-east Australia) and further marked increases are expected due to anthropogenic climate change …
(p 17)
Land Surface
How does land-use change affect climate?
Earth’s climate is strongly affected by the nature of the land-surface, including the vegetation and soil type and the amount of water stored on the land as soil moisture, snow and groundwater.
Vegetation and soils affect the surface albedo, which determines the amount of sunlight absorbed by the land.
The land surface also affects the partitioning of rainfall into evapotranspiration (which cools the surface and moistens the atmosphere) and runoff (which provides much of our freshwater). …
Land-cover change also affects climate change by releasing CO2 to the atmosphere and by modifying the land carbon sink.
[Tropical deforestation] contributes about a fifth of global CO2 emissions and also influences the land-to-atmosphere fluxes of water and energy.
Avoiding deforestation therefore eliminates a significant fraction of anthropogenic CO2 emissions, and maintains areas like the Amazon rainforest which [support] high biodiversity …
Climate Change and the Amazon Rainforest
A key area of concern has been the remaining intact Amazonian rainforest which is susceptible to ‘dieback’ in some climate models due to the combined effects of increasing greenhouse gases and reducing particulate or ‘aerosol’ pollution in the northern hemisphere. …
The drought in Western Amazonia in 2005 [created] a massive carbon source [in the region] against the backdrop of a significant carbon sink in the decades before.
The forests of Amazonia are therefore sensitive to ‘2005- like’ droughts and these are expected to become more common in the 21st century.
(p 19)
A similar story emerges from the analysis of satellite and CO2 flux measurements during the European drought of 2003.
The IPCC AR4 tentatively suggested a link between global warming and the 2003 drought [which] had an enormous impact on the health and functioning of both natural and managed landscapes in the region.
How large are feedbacks linking land-surface and climate?
The response of the land-surface to climatic anomalies feeds back on the climate by changing the fluxes of energy, water and CO2 between the land and the atmosphere. …
In some regions, such as the Sahel, land-atmosphere coupling may be strong enough to support two alternative climate-vegetation states …- one wet and vegetated,
- the other dry and desert-like.
The climate-carbon cycle models reported in the IPCC AR4 reproduced the historical land carbon sink predominantly through `CO2 fertilization’. There is evidence of CO2 fertilization being limited in nitrogen-limited ecosystems …
The IPCC AR4 climate-carbon cycle models also represented a counteracting tendency for CO2 to be released more quickly from the soils as the climate warms [and there] is some suggestion of a slow-down of natural carbon sinks in the recent observational record …
[Also,] strong amplifying land carbon-climate feedback … seems to be consistent with records of the little ice-age period.
Does the land-surface care about the causes of climate change?
CO2 increases affect the land … directly through CO2-fertilization of photosynthesis, and ‘CO2-induced stomatal closure’ which tends to increase plant water-use efficiency [and reduce transpiration]. …
If transpiration is suppressed via higher CO2, the lower evaporative cooling may also lead to higher temperatures.
There is also the potential for significant positive impacts on freshwater resources, but this is still an area of active debate.
By contrast, increases in near surface ozone have strong negative impacts on vegetation by damaging leaves and their photosynthetic capacity.
[Historical] increases in near surface ozone have probably suppressed land carbon uptake and therefore increased the rate of growth of CO2 in the 20th century. ….
Increasing aerosol loadings from around 1950 to 1980, associated predominantly with the burning of sulphurous coal, reduced the amount of sunlight at the surface, which has been coined ‘global dimming’.
[Plants are] more light-efficient if the sunlight is ‘diffuse’ [and aerosol] pollution would certainly have scattered the sunlight, making it more diffuse. …
It seems that [this] ‘diffuse radiation fertilization’ [enhanced] the global land-carbon sink by about a quarter from 1960 to 2000.
This implies that the land carbon sink will decline if we reduce the amount of potentially harmful particulates in the air.
(p 40)
Permafrost and Hydrates
[The] southern boundary of the discontinuous permafrost zone has shifted northward over North America in recent decades.
Rapid degradation and upward movement of the permafrost lower limit has continued on the Tibetan plateau.
[Observations] in Europe have noted permafrost thawing and a substantial increase in the depth of the overlying active layer exposed to an annual freeze/thaw cycle, especially in Sweden.
As permafrost melts and the depth of the active layer deepens, more organic material can potentially start to decay.
If the surface is covered with water, methane-producing bacteria break down the organic matter. …
[If] the thawed soils are exposed to air, carbon dioxide-producing bacteria are involved in the decay process.
Either case is an amplifying feedback to global warming.
[The] magnitude of the feedback represents an important unknown [and] has not been accounted for in any of the IPCC projections.
The total amount of carbon stored in permafrost has been estimated to be around 1672 Gt [which] represents about twice the amount of carbon contained in the atmosphere.
A recent analysis … has found strong direct observational evidence for an acceleration of carbon emissions in association with climate warming from a peat bog overlying permafrost at a site in northern Sweden.
Another amplifying feedback to warming that has recently been observed in high northern latitudes involves the microbial transformation of nitrogen trapped in soils to nitrous oxide. …
Between 500 and 10,000 Gt of carbon are thought to be stored under the sea floor in the form methane hydrates (or clathrates), a crystalline structure of methane gas and water molecules.
Another 7.5 to 400 Gt of carbon are stored in the form of methane hydrates trapped in permafrost.
[Anthropogenic warming raises] the possibility of a catastrophic release of methane from hydrates to the atmosphere.
[The] US Climate Change Science Program [has deemed that it is] very unlikely that such a release would occur this century …
[Athough,] the same assessment deemed it to be very likely that methane sources from hydrate and wetland emissions would increase as the climate warmed.
(p 21)
