Observational evidence of global climate change includes increases in global and regional surface temperature, extent of snow/ice, sea-level rise, hurricanes, and ocean heat content; polar amplification of anthropogenic warming (Arctic); and possible alterations in the thermohaline/meridional overturning circulation. Complex feedbacks modify the response of climate to external factors; climate sensitivity determines this response. Some inertia exists in the climate system that can eventually lead to abrupt changes if the climate system reaches a tipping point. Climate models are used to investigate future climate change and provide predictions. One of the difficulties in making predictions is disentangling natural from anthropogenic effects.

Introduction

As seen in Chapter 2, emissions of CO2 and other greenhouse gases from the burning of fossil fuels, the clearing of forests, and other human activities have been accelerating over the past century. Earth's climate is particularly sensitive to greenhouse gases because they remain in the atmosphere and modify the radiation balance of the planet for decades or even centuries. The consequence is an increase in global surface temperature (or global warming) and, more broadly, climate changes. Rising temperatures can have a profound impact on many aspects of Earth's climate, in particular on the heat content of the ocean and the melting of glaciers and polar ice. Ongoing warming also has the potential to produce more extreme heat and drought, rising sea levels, and more intense tropical storms.

Many signs associated with global warming are already evident, including rising worldwide temperatures and accelerated melting of Arctic ice.