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| Read also the new edition 2022 IPCC Assessment Report on Climate Change |
The source document for this Digest states:
Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.
Changes in climate occur as a result of both internal variability within the climate system and external factors (both natural and anthropogenic).
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The source document for this Digest states:
The influence of external factors on climate can be broadly compared using the concept of radiative forcing8 . A positive radiative forcing, such as that produced by increasing concentrations of greenhouse gases, tends to warm the surface. A negative radiative forcing, which can arise from an increase in some types of aerosols (microscopic airborne particles) tends to cool the surface. Natural factors, such as changes in solar output or explosive volcanic activity, can also cause radiative forcing.
Characterisation of these climate forcing agents and their changes over time (see Figure 2) is required to understand past climate changes in the context of natural variations and to project what climate changes could lie ahead. Figure 3 shows current estimates of the radiative forcing due to increased concentrations of atmospheric constituents and other mechanisms.
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The source document for this Digest states:
Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activities.
The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750. The present CO2 concentration has not been exceeded during the past 420,000 years and likely7 not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.
About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.
Currently the ocean and the land together are taking up about half of the anthropogenic CO2 emissions. On land, the uptake of anthropogenic CO2 very likely7 exceeded the release of CO2 by deforestation during the 1990s.
The rate of increase of atmospheric CO2 concentration has been about 1.5 ppm9 (0.4%) per year over the past two decades. During the 1990s the year to year increase varied from 0.9 ppm (0.2%) to 2.8 ppm (0.8%). A large part of this variability is due to the effect of climate variability(e.g., El Niño events) on CO2 uptake and release by land and oceans. Links...
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Non-CO 2 greenhouse gases
The atmospheric concentration of methane (CH4) has increased by 1060 ppb9 (151%) since 1750 and continues to increase. The present CH4 concentration has not been exceeded during the past 420,000 years. The annual growth in CH4 concentration slowed and became more variable in the 1990s, compared with the 1980s. Slightly more than half of current CH4 emissions are anthropogenic (e.g., use of fossil fuels, cattle, rice agriculture and landfills). In addition, carbon monoxide (CO) emissions have recently been identified as a cause of increasing CH4 concentration.
The atmospheric concentration of nitrous oxide (N2O) has increased by 46 ppb9 (17%) since 1750 and continues to increase. The present N2O concentration has not been exceeded during at least the past thousand years. About a third of current N2O emissions are anthropogenic (e.g., agricultural soils, cattle feed lots and chemical industry).
Since 1995, the atmospheric concentrations of many of those halocarbon gases (CFCs) that are both ozone-depleting and greenhouse gases (e.g., CFCl3 and CF2Cl2), are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol and its Amendments. Their substitute compounds (e.g., CHF2Cl and CF3CH2F) and some other synthetic compounds (e.g., perfluorocarbons (PFCs) and sulphur hexafluoride (SF6)) are also greenhouse gases, and their concentrations are currently increasing.
The radiative forcing due to increases of the well-mixed greenhouse gases from 1750 to 2000 is estimated to be 2.43 Wm-2:
- 1.46 Wm-2 from CO2;
- 0.48 Wm-2 from CH4;
- 0.34 Wm-2 from the halocarbons;
- and 0.15 Wm-2 from N2O.
(See Figure 3, where the uncertainties are also illustrated.)
The observed depletion of the stratospheric ozone(O3) layer from 1979 to 2000 is estimated to have caused a negative radiative forcing (–0.15 Wm-2). Assuming full compliance with current halocarbon regulations, the positive forcing of the halocarbons will be reduced as will the magnitude of the negative forcing from stratospheric ozone depletion as the ozone layer recovers over the 21st century.
The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3-forming gases. This corresponds to a positive radiative forcing of 0.35 Wm-2. O3 forcing varies considerably by region and responds much more quickly to changes in emissions than the long-lived greenhouse gases, such as CO2. Links...
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The source document for this Digest states:
Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing.
The major sources of anthropogenic aerosols are fossil fuel and biomass burning. These sources are also linked to degradation of air quality and acid deposition.
Since the SAR4, significant progress has been achieved in better characterising the direct radiative roles of different types of aerosols. Direct radiative forcing is estimated to be -0.4 Wm-2 for sulphate, -0.2 Wm-2 for biomass burning aerosols, -0.1 Wm-2 for fossil fuel organic carbon and +0.2 Wm-2 for fossil fuel black carbon aerosols. There is much less confidence in the ability to quantify the total aerosol direct effect, and its evolution over time, than that for the gases listed above. Aerosols also vary considerably by region and respond quickly to changes in emissions.
In addition to their direct radiative forcing, aerosols have an indirect radiative forcing through their effects on clouds. There is now more evidence for this indirect effect, which is negative, although of very uncertain magnitude.
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The source document for this Digest states:
Natural factors have made small contributions to radiative forcing over the past century.
The radiative forcing due to changes in solar irradiance for the period since 1750 is estimated to be about +0.3 Wm-2, most of which occurred during the first half of the 20th century. Since the late 1970s, satellite instruments have observed small oscillations due to the 11-year solar cycle. Mechanisms for the amplification of solar effects on climate have been proposed, but currently lack a rigorous theoretical or observational basis.
Stratospheric aerosols from explosive volcanic eruptions lead to negative forcing, which lasts a few years. Several major eruptions occurred in the periods 1880 to 1920 and 1960 to 1991.
The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades.
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The source document for this Digest states:
Confidence in the ability of models to project future climate has increased.
Complex physically-based climate models are required to provide detailed estimates of feedbacks and of regional features. Such models cannot yet simulate all aspects of climate (e.g., they still cannot account fully for the observed trend in the surface-troposphere temperature difference since 1979) and there are particular uncertainties associated with clouds and their interaction with radiation and aerosols. Nevertheless, confidence in the ability of these models to provide useful projections of future climate has improved due to their demonstrated performance on a range of space and time-scales.
- Understanding of climate processes and their incorporation in climate models have improved, including water vapour, sea-ice dynamics, and ocean heat transport.
- Some recent models produce satisfactory simulations of current climate without the need for non-physical adjustments of heat and water fluxes at the ocean-atmosphere interface used in earlier model
- Simulations that include estimates of natural and anthropogenic forcing reproduce the observed large-scale changes in surface temperature over the 20th century (Figure 4). However, contributions from some additional processes and forcings may not have been included in the models. Nevertheless, the large-scale consistency between models and observations can be used to provide an independent check on projected warming rates over the next few decades under a given emissions scenario.
- Some aspects of model simulations of ENSO, monsoons and the North Atlantic Oscillation, as well as selected periods of past climate, have improved
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The source document for this Digest states:
Further action is required to address remaining gaps in information and understanding.
Further research is required to improve the ability to detect, attribute and understand climate change, to reduce uncertainties and to project future climate changes. In particular, there is a need for additional systematic and sustained observations, modelling and process studies. A serious concern is the decline of observational networks. The following are high priority areas for action.
- Systematic observations and reconstructions:
- Reverse the decline of observational networks in many parts of the world.
- Sustain and expand the observational foundation for climate studies by providing accurate, long-term, consistent data including implementation of a strategy for integrated global observations.
- Enhance the development of reconstructions of past climate periods.
- Improve the observations of the spatial distribution of greenhouse gases and aerosols.
- Modelling and process studies:
- Improve understanding of the mechanisms and factors leading to changes in radiative forcing.
- Understand and characterise the important unresolved processes and feedbacks, both physical and biogeochemical, in the climate system.
- Improve methods to quantify uncertainties of climate projections and scenarios, including long-term ensemble simulations using complex models.
- Improve the integrated hierarchy of global and regional climate models with a focus on the simulation of climate variability, regional climate changes and extreme events.
- Link more effectively models of the physical climate and the biogeochemical system, and in turn improve coupling with descriptions of human activities.
Cutting across these foci are crucial needs associated with strengthening international co-operation and co-ordination in order to better utilise scientific, computational and observational resources. This should also promote the free exchange of data among scientists. A special need is to increase the observational and research capacities in many regions, particularly in developing countries. Finally, as is the goal of this assessment, there is a continuing imperative to communicate research advances in terms that are relevant to decision making.
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The source document for this Digest states:
There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
The SAR4 concluded: “The balance of evidence suggests a discernible human influence on global climate”. That report also noted that the anthropogenic signal was still emerging from the background of natural climate variability. Since the SAR4, progress has been made in reducing uncertainty, particularly with respect to distinguishing and quantifying the magnitude of responses to different external influences. Although many of the sources of uncertainty identified in the SAR4 still remain to some degree, new evidence and improved understanding support an updated conclusion.
- There is a longer and more closely scrutinised temperature record and new model estimates of variability. The warming over the past 100 years is very unlikely7 to be due to internal variability alone, as estimated by current models. Reconstructions of climate data for the past 1,000 years (Figure 1b) also indicate that this warming was unusual and is unlikely7 to be entirely natural in origin.
- There are new estimates of the climate response to natural and anthropogenic forcing, and new detection techniques have been applied. Detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35 to 50 years.
- Simulations of the response to natural forcings alone (i.e., the response to variability in solar irradiance and volcanic eruptions) do not explain the warming in the second half of the 20th century (see for example Figure 4a). However, they indicate that natural forcings may have contributed to the observed warming in the first half of the 20th century.
- The warming over the last 50 years due to anthropogenic greenhouse gases can be identified despite uncertainties in forcing due to anthropogenic sulphate aerosol and natural factors (volcanoes and solar irradiance). The anthropogenic sulphate aerosol forcing, while uncertain, is negative over this period and therefore cannot explain the warming. Changes in natural forcing during most of this period are also estimated to be negative and are unlikely7 to explain the warming.
- Detection and attribution studies comparing model simulated changes with the observed record can now take into account uncertainty in the magnitude of modelled response to external forcing, in particular that due to uncertainty in climate sensitivity.
- Most of these studies find that, over the last 50 years, the estimated rate and magnitude of warming due to increasing concentrations of greenhouse gases alone are comparable with, or larger than, the observed warming. Furthermore, most model estimates that take into account both greenhouse gases and sulphate aerosols are consistent with observations over this period.
- The best agreement between model simulations and observations over the last 140 years has been found when all the above anthropogenic and natural forcing factors are combined, as shown in Figure 4c. These results show that the forcings included are sufficient to explain the observed changes, but do not exclude the possibility that other forcings may also have contributed.
In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely7 to have been due to the increase in greenhouse gas concentrations.
Furthermore, it is very likely7 that the 20th century warming has contributed significantly to the observed sea level rise, through thermal expansion of sea water and widespread loss of land ice. Within present uncertainties, observations and models are both consistent with a lack of significant acceleration of sea level rise during the 20th century.
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