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The global mean radiative forcing of the climate system for the year 2000, relative to 1750"Figure 3: Many external factors force climate change. These radiative forcings arise from changes in the atmospheric composition, alteration of surface reflectance by land use, and variation in the output of the sun. Except for solar variation, some form of human activity is linked to each. The rectangular bars represent estimates of the contributions of these forcings - some of which yield warming, and some cooling. Forcing due to episodic volcanic events, which lead to a negative forcing lasting only for a few years, is not shown. The indirect effect of aerosols shown is their effect on the size and number of cloud droplets. A second indirect effect of aerosols on clouds, namely their effect on cloud lifetime, which would also lead to a negative forcing, is not shown. Effects of aviation on greenhouse gases are included in the individual bars. The vertical line about the rectangular bars indicates a range of estimates, guided by the spread in the published values of the forcings and physical understanding. Some of the forcings possess a much greater degree of certainty than others. A vertical line without a rectangular bar denotes a forcing for which no best estimate can be given owing to large uncertainties. The overall level of scientific understanding for each forcing varies considerably, as noted. Some of the radiative forcing agents are well mixed over the globe, such as CO2, thereby perturbing the global heat balance. Others represent perturbations with stronger regional signatures because of their spatial distribution, such as aerosols. For this and other reasons, a simple sum of the positive and negative bars cannot be expected to yield the net effect on the climate system. The simulations of this assessment report (for example, Figure 5) indicate that the
estimated net effect of these perturbations is to have warmed the global climate since
1750. Related publication:
Other Figures & Tables on this publication: Adaptive Capacity, Vulnerability, and Key Concerns by Region Facts on environmental matters Table SPM.1 Estimates of potential global greenhouse gas emission reductions in 2010 and in 2020 The Emissions Scenarios from the Special Report on Emissions Scenarios (SRES) Expert Links on Climate Change Expert Links on Climate Change Expert Links on Climate Change Expert Links on Climate Change Expert Links on Climate Change Expert Links on Climate Change Expert Links on Climate Change Footnotes for the Summary for Policymakers of IPCC Working Group III Footnotes for Table 1 of IPCC SPM WG II Footnotes for the Summary for Policymakers of IPCC Working Group 1 Footnotes for the Summary for Policymakers of IPCC Working Group II Variations of the Earth's surface temperature for : Indicators of the human influence on the atmosphere during the Industrial Era The global mean radiative forcing of the climate system for the year 2000, relative to 1750 Simulated annual global mean surface temperatures The global climate of the 21st Century Documented Climate Change Impacts Figure SPM-2 - Reasons for Concern Figure SPM-3 - Projected Changes in Annual Runoff Footnotes to Table SPM-1 (Question 6.2.2) Many external factors force climate change The annual mean change of the temperature (colour shading) and its range (isolines) Analysis of inter-model consistency in regional relative warming |
