Climate forcing growth rates: doubling down on our Faustian bargain

Rahmstorf et al 's (2012) conclusion that observed climate change is comparable to projections, and in some cases exceeds projections, allows further inferences if we can quantify changing climate forcings and compare those with projections. The largest climate forcing is caused by well-mixed long-lived greenhouse gases. Here we illustrate trends of these gases and their climate forcings, and we discuss implications. We focus on quantities that are accurately measured, and we include comparison with fixed scenarios, which helps reduce common misimpressions about how climate forcings are changing.

Annual fossil fuel CO₂ emissions have shot up in the past decade at about 3% yr^-1, double the rate of the prior three decades (figure 1). The growth rate falls above the range of the IPCC (2001) 'Marker' scenarios, although emissions are still within the entire range considered by the IPCC SRES (2000). The surge in emissions is due to increased coal use (blue curve in figure 1), which now accounts for more than 40% of fossil fuel CO₂ emissions.

Figure 1. CO₂ annual emissions from fossil fuel use and cement manufacture, an update of figure 16 of Hansen (2003) using data of British Petroleum (BP 2012) concatenated with data of Boden et al (2012).

The resulting annual increase of atmospheric CO₂ (12-month running mean) has grown from less than 1 ppm yr^-1 in the early 1960s to an average ~2 ppm yr^-1 in the past decade (figure 2). Although CO₂ measurements were not made at sufficient locations prior to the early 1980s to calculate the global mean change, the close match of global and Mauna Loa data for later years suggests that Mauna Loa data provide a good approximation of global change (figure 2), thus allowing a useful estimate of annual global change beginning with the initiation of Mauna Loa measurements in 1958 by Keeling et al (1973).

Figure 2. Annual increase of CO₂ based on data from the NOAA Earth System Research Laboratory (ESRL 2012). CO₂ change and global temperature change are 12-month running means of differences for the same month of consecutive years. Nino index (Nino3.4 area) is 12-month running mean. Both temperature indices use data from Hansen et al (2010). Annual mean CO₂ amount in 1958 was 315 ppm (Mauna Loa) and in 2012 was 394 ppm (Mauna Loa) and 393 ppm (Global).

Interannual variability of CO₂ growth is correlated with ENSO (El Nino Southern Oscillation) variations of tropical temperatures (figure 2). Ocean–atmosphere CO₂ exchange is affected by ENSO (Chavez et al 1999), but ENSO seems to have a greater impact on atmospheric CO₂ via the terrestrial carbon cycle through effects on the water cycle, temperature, and fire, as discussed in a large body of literature (referenced, e.g., by Schwalm et al 2011). In addition, volcanoes, such as the 1991 Mount Pinatubo eruption, slow the increase of atmospheric CO₂ (Rothenberg et al 2012), at least in part because photosynthesis is enhanced by the increased proportion of diffuse sunlight (Gu et al 2003, Mercado et al 2009). Watson (1997) suggests that volcanic dust deposited on the ocean surface may also contribute to CO₂ uptake by increasing ocean productivity.

An important question is whether ocean and terrestrial carbon sinks will tend to saturate as human-made CO₂ emissions continue. Piao et al (2008) and Zhao and Running (2010) suggest that there already may be a reduction of terrestrial carbon uptake, while Le Quéréet al (2007) and Schuster and Watson (2007) find evidence of decreased carbon uptake in the Southern Ocean and North Atlantic Ocean, respectively. However, others (Knorr 2009, Sarmiento et al 2010, Ballantyne et al 2012) either cast doubt on the reality of a reduced uptake strength or find evidence for increased uptake.

An informative presentation of CO₂ observations is the ratio of annual CO₂ increase in the air divided by annual fossil fuel CO₂ emissions (Keeling et al 1973), the 'airborne fraction' (figure 3, right scale). An alternative definition of airborne fraction includes in the denominator of this ratio an estimated net anthropogenic CO₂ source from changes in land use, but this latter term is much more uncertain than the two terms involved in the Keeling et al (1973) definition. For example, analysis by Harris et al (2012) reveals a range as high as a factor of 2–4 in estimates of recent land use emissions; see also the discussion by Sarmiento et al (2010). However, note that the airborne fraction becomes smaller when estimated land use emissions are included, with the uptake fraction (one minus airborne fraction) typically greater than 0.5.

Figure 3. Fossil fuel CO₂ emissions (left scale) and airborne fraction, i.e., the ratio of observed atmospheric CO₂ increase to fossil fuel CO₂ emissions. Final three points are 5-, 3- and 1-year means.

The simple Keeling airborne fraction, clearly, is not increasing (figure 3). Thus the net ocean plus terrestrial sink for carbon emissions has increased by a factor of 3–4 since 1958, accommodating the emissions increase by that factor.

Remarkably, and we will argue importantly, the airborne fraction has declined since 2000 (figure 3) during a period without any large volcanic eruptions. The 7-year running mean of the airborne fraction had remained close to 60% up to 2000, except for the period affected by Pinatubo. The airborne fraction is affected by factors other than the efficiency of carbon sinks, most notably by changes in the rate of fossil fuel emissions (Gloor et al 2010). However, it is the dependence of the airborne fraction on fossil fuel emission rate that makes the post-2000 downturn of the airborne fraction particularly striking. The change of emission rate in 2000 from 1.5% yr^-1 to 3.1% yr^-1 (figure 1), other things being equal, would have caused a sharp increase of the airborne fraction (the simple reason being that a rapid source increase provides less time for carbon to be moved downward out of the ocean's upper layers).

A decrease in land use emissions during the past decade (Harris et al 2012) could contribute to the decreasing airborne fraction in figure 3, although Malhi (2010) presents evidence that tropical forest deforestation and regrowth are approximately in balance, within uncertainties. Land use change can be only a partial explanation for the decrease of the airborne fraction; something more than land use change seems to be occurring.

We suggest that the huge post-2000 increase of uptake by the carbon sinks implied by figure 3 is related to the simultaneous sharp increase in coal use (figure 1). Increased coal use occurred primarily in China and India (Boden et al 2012; BP 2012; see graphs at www.columbia.edu/~mhs119/Emissions/Emis_moreFigs/). Satellite radiance measurements for July–December, months when desert dust does not dominate aerosol amount, yield an increase of aerosol optical depth in East Asia of about 4% yr^-1 during 2000–2006 (van Donkelaar et al 2008). Associated gaseous and particulate emissions increased rapidly after 2000 in China and India (Lu et al 2011, Tian et al 2010). Some decrease of the sulfur component of emissions occurred in China after 2006 as wide application of flue-gas desulfurization began to be initiated (Lu et al 2010), but this was largely offset by continuing emission increases from India (Lu et al 2011).

We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large increase of carbon uptake by the combined terrestrial and ocean carbon sinks. One mechanism by which fossil fuel emissions increase carbon uptake is by fertilizing the biosphere via provision of nutrients essential for tissue building, especially nitrogen, which plays a critical role in controlling net primary productivity and is limited in many ecosystems (Gruber and Galloway 2008). Modeling (e.g., Thornton et al 2009) and field studies (Magnani et al 2007) confirm a major role of nitrogen deposition, working in concert with CO₂ fertilization, in causing a large increase in net primary productivity of temperate and boreal forests. Sulfate aerosols from coal burning also might increase carbon uptake by increasing the proportion of diffuse insolation, as noted above for Pinatubo aerosols, even though the total solar radiation reaching the surface is reduced.

Thus we see the decreased CO₂ airborne fraction since 2000 as sharing some of the same causes as the decreased airborne fraction after the Pinatubo eruption (figure 3). CO₂ fertilization is likely the major effect, as a plausible addition of 5 TgN yr^-1 from fossil fuels and net ecosystem productivity of 200 kgC kgN^-1 (Magnani et al 2007, 2008) yields an annual carbon drawdown of 1 GtC yr^-1, which is of the order of what is needed to explain the post-2000 anomaly in airborne CO₂. However, an aerosol-induced increase of diffuse radiation might also contribute. Although tropospheric aerosol properties are not accurately monitored, there are suggestions of an upward trend of stratospheric background aerosols since 2000 (Hofmann et al 2009, Solomon et al 2011), which could be a consequence of more tropospheric aerosols at low latitudes where injection of tropospheric air into the stratosphere occurs (Holton et al 1995). We discuss climate implications of the reduced CO₂ airborne fraction after presenting data for other greenhouse gases.

Atmospheric CH₄ is increasing more slowly than in IPCC scenarios (figure 4), which were defined more than a decade ago (IPCC 2001). However, after remaining nearly constant for several years, CH₄ has increased during the past five years, pushing slightly above the level that was envisaged in the Alternative Scenario of Hansen et al (2000). Reduction of CH₄, besides slowdown in CO₂ growth in the twenty first century and a decline of CO₂ in the twenty second century, is a principal requirement to achieve a low climate forcing that stabilizes climate, in part because CH₄ also affects tropospheric ozone and stratospheric water vapor. The Alternative Scenario, defined in detail by Hansen and Sato (2004), keeps maximum global warming at ~1.5 °C relative to 1880–1920, under the assumption that fast-feedback climate sensitivity is ~3 °C for doubled CO₂ (Hansen et al 2007). The Alternative Scenario allows CO₂ to reach 475 ppm in 2100 before declining slowly; this scenario assumes that reductions of non-CO₂ greenhouse gases and black carbon aerosols can be achieved sufficient to balance the warming effect of likely future decreases of reflective aerosols.

Figure 4. Observed atmospheric CH₄ amount and scenarios for twenty first century. Alternative scenario (Hansen et al 2000, Hansen and Sato 2004) yields maximum global warming ~1.5 °C above 1880–1920. Other scenarios are from IPCC (2001). Forcing on right hand scale is adjusted forcing, Fa, relative to values in 2000 (Hansen et al 2007).

There are anthropogenic sources of CH₄ that potentially could be reduced, indeed, the leveling off of CH₄ amount during the past 20 years seems to have been caused by decreased venting in oil fields (Simpson et al 2012), but the feasibility of overall CH₄ reduction also depends on limiting global warming itself, because of the potential for amplifying climate-CH₄ feedbacks (Archer et al 2009, Koven et al 2011). Furthermore, reduction of atmospheric CH₄ might become problematic if unconventional mining of gas, such as 'hydro-fracking', expands widely (Cipolla 2009), as discussed further below.

The growth rate for the total climate forcing by well-mixed greenhouse gases has remained below the peak values reached in the 1970s and early 1980s, has been relatively stable for about 20 years, and is falling below IPCC (2001) scenarios (figure 5). However, the greenhouse gas forcing is growing faster than in the Alternative Scenario. MPTGs and OTGs in figure 5 are Montreal Protocol Trace Gases and Other Trace Gases (Hansen and Sato 2004).

Figure 5. Five-year mean of the growth rate of climate forcing by well-mixed greenhouse gases, an update of figure 4 of Hansen and Sato (2004). Forcing calculations use equations of Hansen et al (2000). The moderate uncertainties in radiative calculations affect the scenarios and actual greenhouse gas results equally and thus do not alter the conclusion that the actual forcing falls below that of the IPCC scenarios.

If greenhouse gases were the only climate forcing, we would be tempted to infer from Rahmstorf's conclusion (that actual climate change has exceeded IPCC projections) and our conclusion (that actual greenhouse gas forcings are slightly smaller than IPCC scenarios) that actual climate sensitivity is on the high side of what has generally been assumed. Although that may be a valid inference, the evidence is weakened by the fact that other climate forcings are not negligible in comparison to the greenhouse gases and must be accounted for.

Natural forcings, by changing solar irradiance and volcanic aerosols, are well-measured since the late 1970s and included in most IPCC (2007) climate simulations. The difficulty is human-made aerosols. Aerosols are readily detected in satellite observations, but determination of their climate forcing requires accurate knowledge of changes in aerosol amount, size distribution, absorption and vertical distribution on a global basis—as well as simultaneous data on changes in cloud properties to allow inference of the indirect aerosol forcing via induced cloud changes. Unfortunately, the first satellite mission capable of measuring the needed aerosol characteristics (Aerosol Polarimetry Sensor on the Glory satellite, (Mishchenko et al 2007)) suffered a launch failure and as yet there are no concrete plans for a replacement mission.

The human-made aerosol climate forcing thus remains uncertain. IPCC (2007) concludes that aerosols are a negative (cooling) forcing, probably between -0.5 and -2.5 W m^-2. Hansen et al (2011), based mainly on analysis of Earth's energy imbalance, derive an aerosol forcing -1.6 ± 0.3 W m^-2, consistent with an analysis of Murphy et al (2009) that suggests an aerosol forcing about -1.5 W m^-2 (see discussion in Hansen et al (2011)). This large negative aerosol forcing reduces the net climate forcing of the past century by about half (IPCC 2007; figure 1 of Hansen et al 2011). Coincidentally, this leaves net climate forcing comparable to the CO₂ forcing alone.

Reduction of the net human-made climate forcing by aerosols has been described as a 'Faustian bargain' (Hansen and Lacis 1990, Hansen 2009), because the aerosols constitute deleterious particulate air pollution. Reduction of the net climate forcing by half will continue only if we allow air pollution to build up to greater and greater amounts. More likely, humanity will demand and achieve a reduction of particulate air pollution, whereupon, because the CO₂ from fossil fuel burning remains in the surface climate system for millennia, the 'devil's payment' will be extracted from humanity via increased global warming.

So is the new data we present here good news or bad news, and how does it alter the 'Faustian bargain'? At first glance there seems to be some good news. First, if our interpretation of the data is correct, the surge of fossil fuel emissions, especially from coal burning, along with the increasing atmospheric CO₂ level is 'fertilizing' the biosphere, and thus limiting the growth of atmospheric CO₂. Also, despite the absence of accurate global aerosol measurements, it seems that the aerosol cooling effect is probably increasing based on evidence of aerosol increases in the Far East and increasing 'background' stratospheric aerosols.

Both effects work to limit global warming and thus help explain why the rate of global warming seems to be less this decade than it has been during the prior quarter century. This data interpretation also helps explain why multiple warnings that some carbon sinks are 'drying up' and could even become carbon sources, e.g., boreal forests infested by pine bark beetles (Kurz et al 2008) and the Amazon rain forest suffering from drought (Lewis et al 2011), have not produced an obvious impact on atmospheric CO₂.

However, increased CO₂ uptake does not necessarily mean that the biosphere is healthier or that the increased carbon uptake will continue indefinitely (Matson et al 2002, Galloway et al 2002, Heimann and Reichstein 2008, Gruber and Galloway 2008). Nor does it change the basic facts about the potential magnitude of the fossil fuel carbon source (figure 6) and the long lifetime of the CO₂ in the surface carbon reservoirs (atmosphere, ocean, soil, biosphere) once the fossil fuels are burned (Archer 2005). Fertilization of the biosphere affects the distribution of the fossil fuel carbon among these reservoirs, at least on the short run, but it does not alter the fact that the fossil carbon will remain in these reservoirs for millennia.

Figure 6. Fossil fuel CO₂ emissions and carbon content (1 ppm atmospheric CO₂~2.12 GtC). Historical emissions are from Boden et al (2012). Estimated reserves and potentially recoverable resources are based on energy content values of Energy Information Administration (EIA 2011), German Advisory Council (GAC 2011), and Global Energy Assessment (GEA 2012). We convert energy content to carbon content using emission factors of Table 4.2 of IPCC (2007) for coal, gas, and conventional oil, and, following IPCC, we use an emission factor of unconventional oil the same as that for coal.

Humanity, so far, has burned only a small portion (purple area in figure 6) of total fossil fuel reserves and resources. Yet deleterious effects of warming are apparent (IPCC 2007), even though only about half of the warming due to gases now in the air has appeared, the remainder still 'in the pipeline' due to the inertia of the climate system (Hansen et al 2011). Already it seems difficult to avoid passing the 'guardrail' of no more than 2 °C global warming that was agreed in the Copenhagen Accord of the United Nations Framework Convention on Climate Change (UNFCCC 2010). And Hansen et al (2008), based primarily on paleoclimate data and evidence of deleterious climate impacts already at 385 ppm CO₂, concluded that an appropriate initial target for CO₂ was 350 ppm, which implied a global temperature limit, relative to 1880–1920 of about 1 °C. What is clear is that most of the remaining fossil fuels must be left in the ground if we are to avoid dangerous human-made interference with climate.

The principal implication of our present analysis probably relates to the Faustian bargain. Increased short-term masking of greenhouse gas warming by fossil fuel particulate and nitrogen pollution represents a 'doubling down' of the Faustian bargain, an increase in the stakes. The more we allow the Faustian debt to build, the more unmanageable the eventual consequences will be. Yet globally there are plans to build more than 1000 coal-fired power plants (Yang and Cui 2012) and plans to develop some of the dirtiest oil sources on the planet (EIA 2011). These plans should be vigorously resisted. We are already in a deep hole—it is time to stop digging.

Acknowledgments

We thank ClimateWorks, Energy Foundation, Gerry Lenfest (Lenfest Foundation), Lee Wasserman (Rockefeller Family Foundation), and Stephen Toben (Flora Family Foundation) for research and communications support.

References

Archer D 2005 Fate of fossil fuel CO2 in geologic time J. Geophys. Res. 110 C09505

Archer D, Buffett B and Brovkin V 2009 Ocean methane hydrates as a slow tipping point in the global carbon cycle Proc. Natl Acad. Sci. 106 20596–601

Ballantyne A P, Alden C B, Miller J B, Tans P P and White J W C 2012 Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years Nature 488 70-–

Boden T A, Marland G and Andres R J 2012 Global, Regional, and National Fossil-Fuel CO₂ Emissions (Oak Ridge, TN: Carbon Dioxide Information and Analysis Center, Oak Ridge National Laboratory, US Department of Energy) doi:10.3334/CDIAC/00001_V2012

BP (British Petroleum) 2012 Statistical Review of World Energy 2012 (www.bp.com/sectionbodycopy.do?categoryId=7500&contentId=7068481)

Chavez F P et al 1999 Biological and chemical response of the Equatorial Pacific Ocean to the 1997–1998 El Nino Science 286 2126–31

Cipolla C L 2009 Modeling production and evaluating fracture performance in unconventional gas reservoirs J. Petrol. Technol. 61 84–90

Earth System Research Laboratory (ESRL) 2012 Trends in Atmospheric Carbon Dioxide (www.esrl.noaa.gov/gmd/ccgg/trends/)

Energy Information Administration (EIA) 2011 International Energy Outlook (www.eia.gov/forecasts/ieo/pdf/0484(2011).pdf, accessed Sep. 2011)

Galloway J N, Cowling E B, Seitzinger S P and Socowlow R H 2002 Reactive nitrogen: too much of a good thing? AMBIO 31 60–3

German Advisory Council on Global Change (GAC) 2011 World in Transition—A Social Contract for Sustainability (www.wbgu.de/en/flagship-reports/fr-2011-a-social-contract/, accessed Oct. 2011)

Global Energy Assessment (GEA) 2012 Toward a Sustainable Future ed T B Johanson et al (Luxemburg: International Institute for Applied Systems Analysis) p 118

Gloor M, Sarmiento J L and Gruber N 2010 What can be learned about carbon cycle climate feedbacks from the CO2 airborne fraction? Atmos. Chem. Phys. 10 7739–51

Gruber N and Galloway J N 2008 An Earth-system perspective of the global nitrogen cycle Nature 451 293–6

Gu L et al 2003 Response of a deciduous forest to the Mount Pinatubo eruption: enhanced photosynthesis Science 299 2035–8

Hansen J 2003 Can we defuse the global warming time bomb? Natural Science posted 1 Aug 2003

Hansen J 2009 Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity (New York: Bloomsbury) p 304

Hansen J E and Lacis A A 1990 Sun and dust versus greenhouse gases: an assessment of their relative roles in global climate change Nature 346 713–9

Hansen J, Ruedy R, Sato M and Lo K 2010 Global surface temperature change Rev. Geophys. 48 RG4004

Hansen J and Sato M 2004 Greenhouse gas growth rates Proc. Natl Acad. Sci. 101 16109–14

Hansen J, Sato M, Kharecha P and von Schuckmann K 2011 Earth's energy imbalance and implications Atmos. Chem. Phys. 11 13421–49

Hansen J, Sato M, Ruedy R, Lacis A and Oinas V 2000 Global warming in the twenty-first century: an alternative scenario Proc. Natl Acad. Sci. 97 9875–80

Hansen J et al 2007 Dangerous human-made interference with climate: a GISS modelE study Atmos. Chem. Phys. 7 2287–312

Hansen J et al 2008 Target atmospheric CO2: where should humanity aim? Open Atmos. Sci. 2 217–31

Harris N L et al 2012 Baseline map of carbon emissions from deforestation in tropical regions Science 336 1573–6

Heimann M and Reichstein M 2008 Terrestrial ecosystem carbon dynamics and climate feedbacks Nature 451 289–92

Hofmann D, Barnes J, O'Niel M, Trudeau M and Neely R 2009 Increase in background stratospheric aerosol observed with lidar at Mauna Loa Observatory and Boulder, Colorado Geophys. Res. Lett. 36 L15808

Holton J R et al 1995 Stratosphere–troposphere exchange Rev. Geophys. 33 403–39

IPCC (Intergovernmental Panel on Climate Change) 2000 Special Report on Emission Scenarios (SRES) ed N Nakicenovic et al (Cambridge: Cambridge University Press) p 599

IPCC (Intergovernmental Panel on Climate Change) 2001 Climate Change 2001: The Scientific Basis ed J T Houghton et al (Cambridge: Cambridge University Press) p 881

IPCC (Intergovernmental Panel on Climate Change) 2007 Climate Change 2007: The Physical Science Basis ed S Solomon et al (Cambridge: Cambridge University Press) p 996

Keeling C D, Whorf T P, Wahlen M and van der Plicht J 1973 Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980 Nature 375 666–70

Knorr W 2009 Is the airborne fraction of anthropogenic CO2 emissions increasing? Geophys. Res. Lett. 36 L21710

Koven C D et al 2011 Permafrost carbon-climate feedbacks accelerte global warming Proc. Natl Acad. Sci. 108 14769–74

Kurz W A et al 2008 Mountain pine beetle and forest carbon feedback to climate change Nature 452 987–90

Le Quéré C et al 2007 Saturation of the Southern Ocean CO2 sink due to recent climate change Science 316 1735–8

Lewis S L, Brando P M, Phillips O L, van der Heijden G M F and Nepstad D 2011 The 2010 Amazon drought Science 331 554

Lu Z, Zhang Q and Streets D G 2011 Sulfur dioxide and primary carbonaceous aerosol emissions in China and India, 1996–2010 Atmos. Chem. Phys. 11 9839–64

Lu Z et al 2010 Sulfur dioxide emissions in China and sulfur trends in East Asia since 2000 Atmos. Chem Phys. 10 6311–31

Magnani F et al 2007 The human footprint in the carbon cycle of temperate and boreal forests Nature 447 848–50

Magnani F et al 2008 Magnani et al. reply Nature 451 E28–9

Malhi Y 2010 The carbon balance of tropical forest regions, 1990–2005 Curr. Opin. Environ. Sustain. 2 237–44

Matson P, Lohse K A and Hall S J 2002 The globalization of nitrogen deposition: consequences for terrestrial ecosystems AMBIO 31 113–9

Mercado L M et al 2009 Impact of changes in diffuse radiation on the global land carbon sink Nature 458 1014–7

Mishchenko M I et al 2007 Accurate monitoring of terrestrial aerosols and total solar irradiance: introducing the Glory mission Bull. Am. Meteorol. Soc. 88 677–91

Murphy D M et al 2009 An observationally based energy balance for the Earth since 1950 J. Geophys. Res. 114 D17107

Piao S et al 2008 Net carbon dioxide losses of northern ecosystems in response to autumn warming Nature 451 49–52

Rahmstorf S, Foster G and Cazenave A 2012 Comparing climate projections to observations up to 2011 Environ. Res. Lett. 7 044035

Rothenberg D et al 2012 Volcano impacts on climate and biogeochemistry in a coupled carbon-climate model Earth Syst. Dyn. Discuss. 3 279–323

Sarmiento J L et al 2010 Trends and regional distributions of land and ocean carbon sinks Biogeosciences 7 2351–67

Schuster U and Watson A J 2007 A variable and decreasing sink for atmospheric CO2 in the North Atlantic J. Geophys. Res. 112 C11006

Schwalm C R et al 2011 Does terrestrial drought explain global CO2 flux anomalies induced by El Nino? Biogeosciences 8 2493–506

Simpson I J et al 2012 Long-term decline of global atmospheric ethane concentrations and implications for methane Nature 488 490–4

Solomon S et al 2011 The persistently variable 'background' stratospheric aerosol layer and global climate change Science 333 866–70

Thornton P E et al 2009 Carbon–nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model Biogeosciences 6 2099–120

Tian H Z et al 2010 Trend and characteristics of atmospheric emissions of Hg, As, and Se from coal combustion in China, 1980–2007 Atmos. Chem. Phys. 10 11905–19

UNFCCC (United Nations Framework Convention on Climate Change) 2010 Copenhagen Accord (http://unfccc.int/resource/docs/2009/cop15/eng/11a01.pdf, accessed 25 Nov. 2012)

van Donkelaar A et al 2008 Analysis of aircraft and satellite measurements from Interconinental Chemical Transport Experiment (INTEX-B) to quantify long-range transport of East Asian sulfur to Canada Atmos. Chem. Phys. 8 2999–3014

Watson A J 1997 Volcanic iron, CO2, ocean productivity and climate Nature 385 587–8

Yang A and Cui Y 2012 Global coal risk assessment: data analysis and market research WRI Working Paper (Washington, DC: World Resources Institute) (www.wri.org/publication/global-coal-risk-assessment)

Zhao M and Running S W 2010 Drought-induced reduction in global terrestrial net primary production from 2000 through 2009 Science 329 940–3