Greenhouse Gas Radiative Forcing Research Articles

Increase of upper troposphere/lower stratosphere wave baroclinicity during the second half of the 20th century

Abstract. A strengthening of the equatorward temperature gradient in the upper troposphere/lower stratosphere (UTLS), at subtropics and midlatitudes, is consistently reproduced in several modelling studies of the atmospheric response to the increase of greenhouse gas radiative forcing. Some of those studies suggest an increase of the baroclinicity in the UTLS region because of the enhanced meridional temperature gradient. This study presents observational evidence of an increase of the baroclinic wave components of UTLS circulation (UTLS wave baroclinicity), during the second half of the 20th century. The evidence is given by significant positive trends in the energy of baroclinic normal modes of the NCEP/NCAR reanalysis, and significant positive trends in the UTLS eddy available potential energy of the NCEP/NCAR, ERA-40, NCEP-2 and JRA-25 reanalyses. Significant positive trends in the frequency of double tropopause events in radiosonde data are also interpreted as a manifestation of an increase of the UTLS wave baroclinicity.

The dynamics of the Indian Ocean sea surface temperature forcing of Sahel drought

Given the pronounced warming in the Indian Ocean sea surface temperature (SST) during the second half of the twentieth century and the empirical relationship between the Indian Ocean SST and Sahel summer precipitation, we investigate the mechanisms underlying this relationship using the GFDL atmospheric model AM2.0 to simulate the equilibrium and transient response to the warming of the Indian Ocean. Equatorial wave dynamics, in particular the westward propagating equatorial Rossby waves, communicates the signal of tropospheric warming and stabilization from the Indian Ocean to the African continent. The stabilization associated with the Rossby wave front acts to suppress the convection. Feedbacks with local precipitation and depletion of moisture amplify the dynamically induced subsidence. While this stabilization mechanism is expected to operate in climate change response, the future prospects for the Sahelian climate under global warming are complicated by the intricate sensitivities to the SSTs from different ocean basins and to the direct radiative forcing of greenhouse gases.

Global Methan Emissions From Wetlands, Rice Paddies, and Lakes

The current concentration of atmospheric methane is 1774±1.8 parts per billion, and it accounts for 18% of total greenhouse gas radiative forcing [Forster et al., 2007]. Atmospheric methane is 22 times more effective, on a per‐unit‐mass basis, than carbon dioxide in absorbing long‐wave radiation on a 100‐year time horizon, and it plays an important role in atmospheric ozone chemistry (e.g., in the presence of nitrous oxides, tropospheric methane oxidation will lead to the formation of ozone). Wetlands are a large source of atmospheric methane, Arctic lakes have recently been recognized as a major source [e.g., Walter et al., 2006], and anthropogenic activities—such as rice agriculture—also make a considerable contribution.

What should protected area managers do to preserve biodiversity in the face of climate change?

Since the 1960s the rate of change of greenhouse gas concentration, radiative forcing and climate warming has been, and will continue to be, more rapid than previously known in geological history. As a result, biomes, species distributions, hydrology and the cryosphere will undergo profound changes. It will threaten some cultural resources and change visitor activities, satisfaction and safety. Protected area management cannot contribute significantly to climate change mitigation, but it can help nature adapt to it, and help society to understand its causes and consequences. The tools for managers are: research and monitoring; awareness and engagement; leading by example; and action on the ground. The management of terrestrial and freshwater ecosystems should focus on the restoration and maintenance of ecosystems, natural processes, biodiversity and abiotic processes, forms and environments that are free of significant impacts from local or regional threats to ecological integrity, cultural resources and human health. Ideally, protected areas should be nested within ecological regions that are permeable for the movement of native species and that contain sufficient habitat for the self-perpetuation of native wildlife populations. Fast and slow migrant native species should be managed to permit orderly ecological shifts that favour the maintenance of high biodiversity. Alien species should be managed to minimize their presence. The management of marine ecosystems should focus on the conservation of biotic resources and sustainable resource use, also free from significant local and regional threats to wild populations and human health. Conversely, parks should neither be manipulated to sequester carbon or to buffer against natural disasters, nor be moved in pursuit of migrating or future ecoregions. Where natural regions guide park establishment, they should not be recast wholesale in anticipation of landscape evolution.

The sensitivity of the Australian summer monsoon to climate forcing during the late Quaternary

A multivariate reduced form model was used to investigate the sensitivity of the Australian summer monsoon, as simulated by the Fast Ocean Atmosphere Model (FOAM), to climate forcing changes over the last 55,000 years. A synthesis of palaeoenvironmental observations was used to constrain FOAM for six time slices: 55, 35, 21, 11, 6, and 0 ka. The reduced form model was constructed to predict wet season rainfall averaged over the Australian monsoon region, the southward extent of the monsoon, and the timing of monsoon onset and decay. The responses were evaluated as a function of solar insolation, vegetation roughness length, global sea level, and global greenhouse gas radiative forcing. Both sea level and solar insolation over the monsoon region are important for predicting the intensity, southward extent, and timing of the monsoon. Conversely, vegetation and greenhouse gas radiative forcing perturbations are responsible for only small proportions of the variation in the simulated monsoon. Increasing sea levels during the Holocene have covered the Sahul Shelf which, when exposed, acted as a thermal trigger for earlier monsoon onset and enhanced intensity. Fluctuating sea levels in concert with varying solar insolation over northern Australia during summer can explain the reduced effectiveness of the monsoon in the Holocene climate, compared with around the time of the first arrival of humans onto mainland Australia (55 ka). During glacial periods, such as at 21 ka, a reduction in the availability of moisture limited the effectiveness of these local forcing factors to control the monsoon.

Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years

The rate of change of climate codetermines the global warming impacts on natural and socioeconomic systems and their capabilities to adapt. Establishing past rates of climate change from temperature proxy data remains difficult given their limited spatiotemporal resolution. In contrast, past greenhouse gas radiative forcing, causing climate to change, is well known from ice cores. We compare rates of change of anthropogenic forcing with rates of natural greenhouse gas forcing since the Last Glacial Maximum and of solar and volcanic forcing of the last millennium. The smoothing of atmospheric variations by the enclosure process of air into ice is computed with a firn diffusion and enclosure model. The 20th century increase in CO(2) and its radiative forcing occurred more than an order of magnitude faster than any sustained change during the past 22,000 years. The average rate of increase in the radiative forcing not just from CO(2) but from the combination of CO(2), CH(4), and N(2)O is larger during the Industrial Era than during any comparable period of at least the past 16,000 years. In addition, the decadal-to-century scale rate of change in anthropogenic forcing is unusually high in the context of the natural forcing variations (solar and volcanoes) of the past millennium. Our analysis implies that global climate change, which is anthropogenic in origin, is progressing at a speed that is unprecedented at least during the last 22,000 years.

Atmospheric Modes of Variability in a Changing Climate

Abstract The response of the atmospheric circulation to an enhanced radiative greenhouse gas forcing in a transient integration with a coupled global climate model is investigated. The spatial patterns of the leading modes of Northern Hemisphere atmospheric variability are shown to change in response to the enhanced forcing. An earlier study showed that the spatial patterns of the leading modes in the Southern Hemisphere changed in response to the enhanced forcing. These changes were associated with changes in the propagation conditions for barotropic Rossby waves. This is, however, not the case for the Northern Hemisphere, where the propagation conditions are unchanged. Other possible mechanisms for the changes in the spatial patterns of the leading modes are discussed.

Long-Term Multi-Gas Scenarios to Stabilise Radiative Forcing - Exploring Costs and Benefits Within an Integrated Assessment Framework

This paper presents a set of multi-gas mitigation scenarios that aim for stabilisation of greenhouse gas radiative forcing in 2150 at levels from 3.7 to 5.3 W/m2. At the moment, non-CO2 gasses (methane, nitrous oxide, PFCs, HFCs and SF6) contribute to about a quarter of the global emissions. The analysis shows that including these non-CO2 gases in mitigation analysis is crucial in formulating a cost-effective response. For stabilisation at 4.5 W/m2, a multi-gas approach leads to 40% lower costs than an approach that would focus at CO2-only. Within the assumptions used in this study, the non-CO2 gasses contribution to total reduction is very large under less stringent targets (up to 60%), but declines under stringent targets. While stabilising at 3.7 W/m2 obviously leads to larger environmental benefits than the 4.5 W/m2 case (temperature increase in 2100 are 1.9 and 2.3oC, respectively), the costs of the lower target are higher (0.80% and 0.34% of GDP in 2100, respectively). Improving knowledge on how future reduction potential for non-CO2 gasses could develop is shown to be a crucial research question.

Twenty-first century Arctic climate change in the CCSM3 IPCC scenario simulations

Arctic climate change in the Twenty-first century is simulated by the Community Climate System Model version 3.0 (CCSM3). The simulations from three emission scenarios (A2, A1B and B1) are analyzed using eight (A1B and B1) or five (A2) ensemble members. The model simulates a reasonable present-day climate and historical climate trend. The model projects a decline of sea-ice extent in the range of 1.4–3.9% per decade and 4.8–22.2% per decade in winter and summer, respectively, corresponding to the range of forcings that span the scenarios. At the end of the Twenty-first century, the winter and summer Arctic mean surface air temperature increases in a range of 4–14°C (B1 and A2) and 0.7–5°C (B1 and A2) relative to the end of the Twentieth century. The Arctic becomes ice-free during summer at the end of the Twenty-first century in the A2 scenario. Similar to the observations, the Arctic Oscillation (AO) is the dominant factor in explaining the variability of the atmosphere and sea ice in the 1870–1999 historical runs. The AO shifts to the positive phase in response to greenhouse gas forcings in the Twenty-first century. But the simulated trends in both Arctic mean sea-level pressure and the AO index are smaller than what has been observed. The Twenty-first century Arctic warming mainly results from the radiative forcing of greenhouse gases. The 1st empirical orthogonal function (explains 72.2–51.7% of the total variance) of the wintertime surface air temperature during 1870–2099 is characterized by a strong warming trend and a “polar amplification”-type of spatial pattern. The AO, which plays a secondary role, contributes to less than 10% of the total variance in both surface temperature and sea-ice concentration.

Modeling impacts of farming management alternatives on CO2, CH4, and N2O emissions: A case study for water management of rice agriculture of China

Since the early 1980s, water management of rice paddies in China has changed substantially, with midseason drainage gradually replacing continuous flooding. This has provided an opportunity to estimate how a management alternative impacts greenhouse gas emissions at a large regional scale. We integrated a process‐based model, DNDC, with a GIS database of paddy area, soil properties, and management factors. We simulated soil carbon sequestration (or net CO2 emission) and CH4 and N2O emissions from China's rice paddies (30 million ha), based on 1990 climate and management conditions, with two water management scenarios: continuous flooding and midseason drainage. The results indicated that this change in water management has reduced aggregate CH4 emissions about 40%, or 5 Tg CH4 yr−1, roughly 5–10% of total global methane emissions from rice paddies. The mitigating effect of midseason drainage on CH4 flux was highly uneven across the country; the highest flux reductions (>200 kg CH4‐C ha−1 yr−1) were in Hainan, Sichuan, Hubei, and Guangdong provinces, with warmer weather and multiple‐cropping rice systems. The smallest flux reductions (<25 kg CH4‐C ha−1 yr−1) occurred in Tianjin, Hebei, Ningxia, Liaoning, and Gansu Provinces, with relatively cool weather and single cropping systems. Shifting water management from continuous flooding to midseason drainage increased N2O emissions from Chinese rice paddies by 0.15 Tg N yr−1 (∼50% increase). This offset a large fraction of the greenhouse gas radiative forcing benefit gained by the decrease in CH4 emissions. Midseason drainage‐induced N2O fluxes were high (>8.0 kg N/ha) in Jilin, Liaoning, Heilongjiang, and Xinjiang provinces, where the paddy soils contained relatively high organic matter. Shifting water management from continuous flooding to midseason drainage reduced total net CO2 emissions by 0.65 Tg CO2‐C yr−1, which made a relatively small contribution to the net climate impact due to the low radiative potential of CO2. The change in water management had very different effects on net greenhouse gas mitigation when implemented across climatic zones, soil types, or cropping systems. Maximum CH4 reductions and minimum N2O increases were obtained when the mid‐season draining was applied to rice paddies with warm weather, high soil clay content, and low soil organic matter content, for example, Sichuan, Hubei, Hunan, Guangdong, Guangxi, Anhui, and Jiangsu provinces, which have 60% of China's rice paddies and produce 65% of China's rice harvest.

The Response of the Southern Hemisphere Atmospheric Circulation to an Enhanced Greenhouse Gas Forcing

Abstract The response of the atmospheric circulation to an enhanced radiative greenhouse gas forcing is investigated. It has been proposed that the response of the climate system to an enhanced forcing projects directly onto the preexisting natural modes of variability. An evaluation of this possibility and in particular of the implications of unchanged flow regimes is performed with a focus on the Southern Hemisphere extratropical atmospheric circulation. Low-pass-filtered mean sea level pressure and geopotential height at 500 and 200 hPa from a transient integration with a coupled global climate model is used. The response to an enhanced forcing projects strongly onto the leading modes of present-day variability, in agreement with other studies. However, the spatial patterns of the leading modes are changed in response to enhanced forcing. The first and second modes of interweekly variability are the Pacific–South American modes, zonal wavenumber-3 wave trains from the central Pacific to the southern Atlantic. In response to the enhanced forcing, the spatial patterns of these modes change, and the wave train extends along a circumpolar path with amplitude also in the Eastern Hemisphere. This change in the spatial patterns is associated with a strengthening of the waveguide for barotropic Rossby waves.

Industrial CO2 emissions as a proxy for anthropogenic influence on lower tropospheric temperature trends

Surface temperature trends during the last two decades show a significant increase which appears to be anthropogenic in origin. We investigate global temperature changes using surface as well as satellite measurements and show that lower tropospheric temperature trends for the period 1979–2001 are spatially correlated to anthropogenic surface CO2 emissions, which we use as a measure of industrialization. Furthermore, temperature trends for the regions not spatially correlated with these CO2 emissions are considerably smaller or even negligible for some of the satellite data. We also show, using the same measure, that two important climate models do not reproduce the geographical climate response to all known forcings as found in the observed temperature trends. We speculate that the observed surface temperature changes might be a result of local surface heating processes and not related to radiative greenhouse gas forcing.

Anthropogenic Forcing and Decadal Climate Variability in Sensitivity Experiments of Twentieth- and Twenty-First-Century Climate

A methodology is formulated to evaluate the possible changes in decadal-timescale (10–20-yr period) surface temperature variability and associated low-frequency fluctuations of anthropogenic forcing and changes in climate base state due to the forcing in simulations of twentieth- and twenty-first-century climate in a global coupled climate model without flux adjustment. The two climate change experiments both start in the year 1900. The first uses greenhouse gas radiative forcing (represented by equivalent CO2) observed during the twentieth century, and extends greenhouse gas forcing to the year 2035 by increasing CO2 1% yr−1 compound after 1990 (CO2-only experiment). The second includes the same greenhouse gas forcing as the first, but adds the effects of time-varying geographic distributions of monthly sulfate aerosol radiative forcing represented by a change in surface albedo (CO2 + sulfates experiment). The climate change experiments are compared with a 135-yr control experiment with no change in external forcing. Climate system responses in the CO2-only and CO2 + sulfates experiments in this particular model are marked not only by greater warming at high latitudes in the winter hemisphere, but also by a global El Niño–like pattern in surface temperature, precipitation, and sea level pressure. This pattern is characterized by a relatively greater increase of SST in the central and eastern equatorial Pacific in comparison with the west, a shift of precipitation maxima from the western Pacific to the central Pacific, mostly decreases of Asian–Australian monsoon strength, lower pressure over the eastern tropical Pacific, deeper midlatitude troughs in the North and South Pacific, and higher pressure over Australasia. Time series analysis of globally averaged temperature and an EOF analysis of surface temperature are consistent with previous results in that enhanced low-frequency variability with periods greater than around 20 yr is introduced into the model coupled climate system with a comparable timescale to the forcing. To examine the possible effects of the associated changes in base state on decadal timescale variability (10–20-yr periods), the surface temperature time series are filtered to retain only variability on that timescale. The El Niño–like pattern of decadal variability seen in the observations is present in each of the model experiments (control, CO2 only, and CO2 + sulfates), but the magnitude decreases significantly in the CO2-only experiment. This decrease is associated with changes in the base-state climate that include a reduction in the magnitude (roughly 5%–20% or more) of wind stress and ocean currents in the upper 100 m in most ocean basins and a weakening of meridional overturning (about 50%) in the Atlantic. These weakened circulation features contribute to decreasing the amplitude of global decadal surface temperature variability as seen in a previous sea-ice sensitivity study with this model. Thus the superposition of low-frequency variability patterns in the radiative forcing increases climate variability for periods comparable to those of the forcing (greater than about 20 yr). However, there are decreases in the amplitude of future decadal (10–20-yr period) variability in these experiments due to changes of the base-state climate as a consequence of increases in that forcing.

Simulation of early 20th century global warming

The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model's climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.

Changes in heat index associated with CO2-induced global warming

Changes in Heat Index (a combined measure of temperature and humidity) associated with global warming are evaluated based on the output from four extended integrations of the GFDL coupled ocean-atmosphere climate model. The four integrations are: a control with constant levels of atmospheric carbon dioxide (CO2), a second integration in which an estimate of the combined radiative forcing of greenhouse gases and sulfate aerosols over the period 1765–2065 is used to force the model, and a third (fourth) integration in which atmospheric CO2$ increases at the rate of 1% per year to double (quadruple) its initial value, and is held constant thereafter. While the spatial patterns of the changes in Heat Index are largely determined by the changes in surface air temperature, increases in atmospheric moisture can substantially amplify the changes in Heat Index over regions which are warm and humid in the Control integration. The regions most prone to this effect include humid regions of the Tropics and summer hemisphere extra-tropics, including the southeastern United States, India, southeast Asia and northern Australia.

Regional climatic warming and associated twentieth century land-cover changes in north-western North America

Twentieth century (1900 to 1990) changes in annual surface air temperature (SAT) are compared with contemporaneous changes in annual ground surface temperature (GST) over an area extending from east of the Cordillera in north-western Canada, to Texas in the south-central United States. One of the largest SAT increases over the past century has occurred in the north-western por- tion of this study area. It also coincides (spatial regression coefficient r = 0.70) with the largest positive GST anomaly in northern North America. However, there are large areas of Alberta, Saskatchewan, Manitoba and southern Northwest Territories that exhibit spatially coherent patterns of differences between SAT and GST warming. These differences appear to be related to twentieth century land-use and land-cover changes. The highest GST warming has been observed in large areas where extensive land-cover changes, such as the clearing of forests, increased forest fire activity, and conversion of prairie grassland to agricultural land, have occurred. It is hypothesized that land-cover change dra- matically alters surface characteristics affecting the radiation budget and energy balance. The parti- tioning between sensible and latent fluxes is altered and the potential for land drying is increased. Cal- culated flux changes associated with land-cover change are comparable in magnitude with greenhouse gas radiative forcing. It therefore appears that through a step change in GST, land-cover changes have contributed to a portion of the observed SAT warming in this region.

Greenhouse gas radiative forcing: Effects of averaging and inhomogeneities in trace gas distribution

Radiative forcing is a useful diagnostic quantity for indicating the size of climate change mechanisms, and for interpreting the results of General Circulation Model (GCM) experiments. It is also ideal for exploring the potential sensitivity of climate change simulations to assumptions regarding, for example, the horizontal and vertical distribution of changes in greenhouse gases, as a guide to whether these effects need to be incorporated into GCMs. This paper examines this issue for a range of greenhouse gases, and also examines the dependence of the forcing on horizontal averaging and the definition of tropopause position. By comparison with calculations at a high horizontal resolution, it is shown that the use of a single global mean profile results in global mean radiative forcing errors of several percent for CO2 and chlorofluorocarbon CFC-12 (CCI2F2); the error is reduced by an order of magnitude or more if three profiles are used, one each representing the tropics and the southern and northern extratropics. Three alternative definitions of tropopause position lead to a spread in forcing of up to 9%. The effect of the spatial variation of a number of greenhouse gases, which are often regarded as well-mixed, is then explored. For methane, three-dimensional distributions are used for present and pre-industrial cases generated from chemical-transport models. The inhomogeneities are shown to have little impact on the forcing, because they are concentrated at the surface. The assumption that methane is well-mixed in both the vertical and horizontal results in errors in the global mean forcing of only 2% compared to the full calculation; at individual locations the error never exceeds 4%. For CFC-11 (CCI3F) and CFC-12, recent satellite data from the Cryogenic Limb Array Etalon Spectrometer are used to define the vertical and latitudinal variation in the stratosphere. For the global mean forcing, the assumption that CFC-11 and CFC-12 are well-mixed in the horizontal and vertical results in errors of less than 4%, which is significantly smaller than current spectroscopic uncertainties. For CFC-11 this error is a factor of two smaller than that implied in recent work, where the forcing from a well-mixed distribution was compared with that of an idealised vertical distribution. However, the zonal variation of the forcing is more seriously affected by the use of the satellite distributions, and errors can reach 30% at high latitudes on assuming a well-mixed distribution. Finally, the distribution of hydrohalocarbons generated from a 2–D chemical model is used, assuming an idealised, predominantly northern hemisphere source distribution. Five gases are included, with lifetimes ranging from 2 to 26 years. The error in global mean radiative forcing arising from the assumption that these gases are well-mixed is 10% for gases with lifetimes above 10 years, but can reach 30% for gases with lifetimes of about 2 years. It is found that almost all of the error is due to the vertical rather than horizontal distribution; the fall-off above the tropopause, determined by the stratospheric lifetime, is an important determinant of the size of the error. The errors in the zonal distribution of the forcing are much larger, and can reach a factor of two for the shorter-lived gases, particularly at high latitudes.

Greenhouse gas radiative forcing: Effects of averaging and inhomogeneities in trace gas distribution

AbstractRadiative forcing is a useful diagnostic quantity for indicating the size of climate change mechanisms, and for interpreting the results of General Circulation Model (GCM) experiments. It is also ideal for exploring the potential sensitivity of climate change simulations to assumptions regarding, for example, the horizontal and vertical distribution of changes in greenhouse gases, as a guide to whether these effects need to be incorporated into GCMs. This paper examines this issue for a range of greenhouse gases, and also examines the dependence of the forcing on horizontal averaging and the definition of tropopause position.By comparison with calculations at a high horizontal resolution, it is shown that the use of a single global mean profile results in global mean radiative forcing errors of several percent for CO2 and chlorofluorocarbon CFC‐12 (CCI2F2); the error is reduced by an order of magnitude or more if three profiles are used, one each representing the tropics and the southern and northern extratropics. Three alternative definitions of tropopause position lead to a spread in forcing of up to 9%.The effect of the spatial variation of a number of greenhouse gases, which are often regarded as well‐mixed, is then explored. For methane, three‐dimensional distributions are used for present and pre‐industrial cases generated from chemical‐transport models. The inhomogeneities are shown to have little impact on the forcing, because they are concentrated at the surface. The assumption that methane is well‐mixed in both the vertical and horizontal results in errors in the global mean forcing of only 2% compared to the full calculation; at individual locations the error never exceeds 4%.For CFC‐11 (CCI3F) and CFC‐12, recent satellite data from the Cryogenic Limb Array Etalon Spectrometer are used to define the vertical and latitudinal variation in the stratosphere. For the global mean forcing, the assumption that CFC‐11 and CFC‐12 are well‐mixed in the horizontal and vertical results in errors of less than 4%, which is significantly smaller than current spectroscopic uncertainties. For CFC‐11 this error is a factor of two smaller than that implied in recent work, where the forcing from a well‐mixed distribution was compared with that of an idealised vertical distribution. However, the zonal variation of the forcing is more seriously affected by the use of the satellite distributions, and errors can reach 30% at high latitudes on assuming a well‐mixed distribution.Finally, the distribution of hydrohalocarbons generated from a 2–D chemical model is used, assuming an idealised, predominantly northern hemisphere source distribution. Five gases are included, with lifetimes ranging from 2 to 26 years. The error in global mean radiative forcing arising from the assumption that these gases are well‐mixed is 10% for gases with lifetimes above 10 years, but can reach 30% for gases with lifetimes of about 2 years. It is found that almost all of the error is due to the vertical rather than horizontal distribution; the fall‐off above the tropopause, determined by the stratospheric lifetime, is an important determinant of the size of the error. The errors in the zonal distribution of the forcing are much larger, and can reach a factor of two for the shorter‐lived gases, particularly at high latitudes.

Climate change and energy policy: The impacts and implications of aerosols

Anthropogenic increases in aerosol concentrations are believed to significantly affect climate, notably by exerting a negative radiative forcing which counteracts, to some extent, the positive radiative forcing of greenhouse gases (GHGs). The potential effects of aerosols and their short atmospheric lifetimes raise issues which are critical to climate policy. This paper isolates the implications of aerosols by treating aerosol emissions as a policy variable separate from GHG emissions, but linked through energy policy. Using a simple climate model, results show that with no GHG abatement, changes in aerosol emissions can significantly affect net radiative forcing, but that the positive forcing of GHGs continues to dominate. Aerosols are also shown to reduce the difference in net radiative forcing between abatement and ‘business-as-usual’ policies, while the ability to reduce this effect through aerosol emissions from energy policy is limited. However, the conclusion that aerosols are beneficial to climate because they counteract greenhouse warming is then questioned; scenarios with high aerosol and GHG emissions are expected to yield both greater uncertainty in mean temperature and a greater likelihood of changes in other climate parameters.

An integrated modeling approach to global carbon and nitrogen cycles: Balancing their budgets

A modeling attempt is presented to describe the global carbon and nitrogen cycle: where it originates, where it stays, and how the global carbon and nitrogen budgets can be balanced. We used integrated but simple box models: a coupled global carbon‐nitrogen cycle and climate model. The global carbon‐nitrogen cycle model describes on a global scale the exchange fluxes between the compartments and their main internal processes and different biogeochemical feedbacks (e.g., for CO2 and N fertilization and for temperature). The coupled model also includes an energy balance climate model, using as input the induced radiative forcing of greenhouse gases and sulfate aerosols. Simulation experiments performed with the coupled model showed that by incorporating only the CO2 fertilization and temperature‐related feedbacks a balanced past carbon budget could not be created. Therefore the N fertilization of the anthropogenic nitrogen depositions for temperate forests, which depends on the fate of the anthropogenic nitrogen inputs, must be included. By varying the N fluxes by river transport, and deposition and denitrification, a balanced past nitrogen budget was obtained. This resulted in an anthropogenic deposition of 40 Tg N yr−1 for the temperate forests and a N fertilization which constitutes 20–50% of the missing carbon sink. Finally, the future fate of anthropogenic emissions of the C and mobilization of the N compounds were projected given the condition of balanced carbon and nitrogen budgets.