Robust Findingsa Key Uncertainties
Observations show Earth’s surface is warming. Globally, 1990s very likely warmest decade in instrumental record (Figure SPM-10b). Atmospheric concentrations of main anthropogenic greenhouse gases (CO2 (Figure SPM-10a), CH4, N2O, and tropospheric O3) increased substantially since the year 1750. Some greenhouse gases have long lifetimes (e.g., CO2, N2O, and PFCs). Most of observed warming over last 50 years likely due to increases in greenhouse gas concentrations due to human activities.

Climate change and attribution Magnitude and character of natural climate variability. Climate forcings due to natural factors and anthropogenic aerosols (particularly indirect effects). Relating regional trends to anthropogenic climate change.
CO2concentrations increasing over 21st century virtually certain to be mainly due to fossil-fuel emissions (Figure SPM-10a). Stabilization of atmospheric CO2 concentrations at 450, 650, or 1,000 ppm would require global anthropogenic CO2 emissions to drop below year 1990 levels, within a few decades, about a century, or about 2 centuries, respectively, and continue to decrease steadily thereafter to a small fraction of current emissions. Emissions would peak in about 1 to 2 decades (450 ppm) and roughly a century (1,000 ppm) from the present. For most SRES scenarios, SO2 emissions (precursor for sulfate aerosols) are lower in the year 2100 compared with year 2000. Future emissions and concentrations of greenhouse gases and aerosols based on models and projections with the SRES and stabilization scenarios Assumptions underlying the wide range(b) of SRES emissions scenarios relating to economic growth, technological progress, population growth, and governance structures (lead to largest uncertainties in projections). Inadequate emission scenarios for ozone and aerosol precursors. Factors in modeling of carbon cycle including effects of climate feedback’s.(b)
Global average surface temperature during 21st century rising at rates very likely without precedent during last 10,000 years (Figure SPM-10b). Nearly all land areas very likely to warm more than the global average, with more hot days and heat waves and fewer cold days and cold waves. Rise in sea level during 21st century that will continue for further centuries. Hydro-logical cycle more intense. Increase in globally averaged precipitation and more intense precipitation events very likely over many areas. Increased summer drying and associated risk of drought likely over most mid-latitude continental interiors. Future changes in global and regional climate based on model projections with SRES scenarios Assumptions associated with a wide rangec of SRES scenarios, as above. Factors associated with model projectionsc, in particular climate sensitivity, climate forcing, and feedback processes especially those involving water vapor, clouds, and aerosols (including aerosol indirect effects). Understanding the probability distribution associated with temperature and sea-level projections. The mechanisms, quantification, time scales, and likelihoods associated with large-scale abrupt/non-linear changes (e.g., ocean thermohaline circulation). Capabilities of models on regional scales especially regarding precipitation) leading to inconsistencies in model projections and difficulties in quantification on local and regional scales.
Projected climate change will have beneficial and adverse effects on both environmental and socio-economic systems, but the larger the changes and the rate of change in climate, the more the adverse effects predominate. The adverse impacts of climate change are expected to fall disproportionately upon developing countries and the poor persons within countries. Ecosystems and species are vulnerable to climate change and other stresses (as illustrated by observed impacts of recent regional temperature changes) and some will be irreversibly damaged or lost. In some mid- to high latitudes, plant productivity (trees and some agricultural crops) would increase with small increases in temperature. Plant productivity would decrease in most regions of the world for warming beyond a few degrees C. Many physical systems are vulnerable to climate change (e.g., the impact of coastal storm surges will be exacerbated by sea-level rise, and glaciers and permafrost will continue to retreat). Regional and global impacts of changes in mean climate and extremes Reliability of local or regional detail in projections of climate change, especially climate extremes. Assessing and predicting response of ecological, social (e.g., impact of vector- and water-borne diseases), and economic systems to the combined effect of climate change and other stresses such as land-use change, local pollution, etc. Identification, quantification, and valuation of damages associated with climate change.
Greenhouse gas emission reduction (mitigation) actions would lessen the pressures on natural and human systems from climate change. Mitigation has costs that vary between regions and sectors. Substantial technological and other opportunities exist for lowering these costs. Efficient emissions trading also reduces costs for those participating in the trading. Emissions constraints on Annex I countries have well-established, albeit varied, “spill-over” eff ects on non-Annex I countries. National mitigation responses to climate change can be more effective if deployed as a portfolio of policies to limit or reduce net greenhouse gas emissions. Adaptation has the potential to reduce adverse effects of climate change and can often produce immediate ancillary benefits, but will not prevent all damages. Adaptation can complement mitigation in a cost-effective strategy to reduce climate change risks; together they can contribute to sustainable development objectives. Inertia in the interacting climate, ecological, and socio-economic systems is a major reason why anticipatory adaptation and mitigation actions are beneficial. Costs and benefits of mitigation and adaptation options Understanding the interactions between climate change and other environmental issues and the related socio-economic implications. The future price of energy, and the cost and availability of low-emissions technology. Identification of means to remove barriers that impede adoption of low-emission technologies, and estimation of the costs of overcoming such barriers. Quantification of costs of unplanned and unexpected mitigation actions with sudden short-term effects. Quantification of mitigation cost estimates generated by different approaches (e.g., bottom-up vs. top-down), including ancillary benefits, technological change, and effects on sectors and regions. Quantification of adaptation costs.
a. In this report, a robust finding for climate change is defined as one that holds under a variety of approaches, methods, models, and assumptions and one that is expected to be relatively unaffected by uncertainties. Key uncertainties in this context are those that, if reduced, may lead to new and robust findings in relation to the questions of this report. This table provides examples and is not an exhaustive list.b. Accounting for these above uncertainties leads to a range of CO2concentrations in the year 2100 between about 490 and 1,260 ppm.c. Accounting for these above uncertainties leads to a range for globally averaged surface temperature increase, 1990-2100, of 1.4 to 5.8oC (Figure SPM-10b) and of globally averaged sea-level rise of 0.09 to 0.88 m.

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