Programmes and specialized agencies. Search this site U. Agency for International Development Carbon dioxide removal Carbon sink Climate action Climate Action Plan Climate change mitigation scenarios Climate engineering Ad4 and political action on climate change Reducing emissions from deforestation and wgg1 degradation Reforestation Urban reforestation. If using citation templates: Integrated design of commercial buildings including technologies, such as intelligent meters that provide feedback and control; solar PV integrated in buildings. Mitigation measures will therefore also be required.
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These changes are expressed in terms of radiative forcing,  which is used to compare how a range of human and natural factors drive warming or cooling influences on global climate. Since the TAR, new observations and related modelling of greenhouse gases, solar activity, land surface properties and some aspects of aerosols have led to improvements in the quantitative estimates of radiative forcing.
Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since and now far exceed pre-industrial values determined from ice cores spanning many thousands of years see Figure SPM.
The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change, while those of methane and nitrous oxide are primarily due to agriculture.
Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the last 10, years large panels and since inset panels. Measurements are shown from ice cores symbols with different colours for different studies and atmospheric samples red lines. The corresponding radiative forcings are shown on the right hand axes of the large panels. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about ppm to ppm  in The atmospheric concentration of carbon dioxide in exceeds by far the natural range over the last , years to ppm as determined from ice cores.
The annual carbon dioxide concentration growth rate was larger during the last 10 years — average: 1. Annual fossil carbon dioxide emissions  increased from an average of 6. Carbon dioxide emissions associated with land-use change are estimated to be 1. The atmospheric concentration of methane in exceeds by far the natural range of the last , years to ppb as determined from ice cores. Growth rates have declined since the early s, consistent with total emissions sum of anthropogenic and natural sources being nearly constant during this period.
It is very likely  that the observed increase in methane concentration is due to anthropogenic activities, predominantly agriculture and fossil fuel use, but relative contributions from different source types are not well determined. The growth rate has been approximately constant since More than a third of all nitrous oxide emissions are anthropogenic and are primarily due to agriculture.
These forcings are now better understood than at the time of the TAR due to improved in situ, satellite and ground-based measurements and more comprehensive modelling, but remain the dominant uncertainty in radiative forcing.
Aerosols also influence cloud lifetime and precipitation. Changes in surface albedo, due to land cover changes and deposition of black carbon aerosols on snow, exert respective forcings of —0. Global average radiative forcing RF estimates and ranges in for anthropogenic carbon dioxide CO2 , methane CH4 , nitrous oxide N2O and other important agents and mechanisms, together with the typical geographical extent spatial scale of the forcing and the assessed level of scientific understanding LOSU.
The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition.
Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness.
Positive forcing tends to warm the surface while negative forcing tends to cool it. In this report, radiative forcing values are for relative to pre-industrial conditions defined at and are expressed in watts per square metre W m—2. See Glossary and Section 2. For example, ppm means molecules of a greenhouse gas per million molecules of dry air. An emission of 1 GtC corresponds to 3. Best estimates are given where available. Assessed uncertainty intervals are not always symmetric about the corresponding best estimate.
AR4 WG1 SPM PDF
Summary for Policymakers
Human and Natural Drivers of Climate Change