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The Fourth Assessment Report

The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC 2007) provides an update of scientific understanding regarding GHG emissions, global warming and the risks of climate change, and the way changes could unfold in the future.

Emissions scenarios and results from climate models (see chart below) estimate that, without policy intervention, temperatures could increase 1º to 5º C by 2100.

Projected climate response to IPCC emissions scenarios
Chart — Combining results from both emissions scenarios and climate models, the colored lines show projected warming from 2000 through 2100 for a range of scenarios. The colored bars indicate the range of warming for each scenario resulting from the uncertainty of converting emissions to global temperature change. The blue line shows historical changes over the past century. (IPCC 2007)

The IPCC also examined a number of stabilization scenarios in which global emissions grow more slowly, peak and then decline in coming decades. In these scenarios GHG concentrations and projected warming stabilize at levels lower than would be expected without policy intervention. Stabilization scenarios rely on policies that place added costs on GHG emissions to reduce future energy use and stimulate the deployment of more efficient technologies. Scenarios resulting in lower concentrations result in reduced climate risks, but, as described below, they require stronger policy intervention.

GHG stabilization in the policy debate

In recognition that GHG concentrations influence long-term risks from climate change, the climate policy debate has shifted from a focus primarily on targets to limit near-term emissions to also include consideration of long-term emissions pathways that ultimately stabilize GHG concentrations.