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Solar variation
Variations in solar activity during the last several centuries based on observations of sunspots and beryllium isotopes.
Variations in solar activity during the last several centuries based on observations of sunspots and beryllium isotopes.
The sun is the ultimate source of essentially all heat in the climate system. The energy output of the sun, which is converted to heat at the Earth's surface, is an integral part of shaping the Earth's climate. On the longest time scales, the sun itself is getting brighter with higher energy output; as it continues its main sequence, this slow change or evolution affects the Earth's atmosphere. It is thought that, early in Earth's history, the sun was too cold to support liquid water at the Earth's surface, leading to what is known as the Faint young sun paradox.

Climate change is one of the most complex issues facing policy-makers today. David G. Victor, a leading expert on environmental policy, takes a fresh look at this issue and persuasively marshals arguments for three distinct approaches to combat the problem, casting each as a presidential speech. A must-read for environmentalists, educators, and anyone else interested in the issue, Climate Change is a most useful reference in the growing public debate about how best to meet this environmental challenge.

Anthropogenic factors are human activities that change the environment and influence climate. In some cases the chain of causality is direct and unambiguous (e.g., by the effects of irrigation on temperature and humidity), while in others it is less clear. Various hypotheses for human-induced climate change have been debated for many years.

The biggest factor of present concern is the increase in CO₂ levels due to emissions from fossil fuel combustion, followed by aerosols (particulate matter in the atmosphere), which exert a cooling effect, and cement manufacture. Other factors, including land use, ozone depletion, animal agriculture and deforestation, also affect climate.

Climate change is with us. A decade ago, it was conjecture. Now the future is unfolding before our eyes. Canada's Inuit see it in disappearing Arctic ice and permafrost. The shantytown dwellers of Latin America and Southern Asia see it in lethal storms and floods. Europeans see it in disappearing glaciers, forest fires and fatal heat waves.

Scientists see it in tree rings, ancient coral and bubbles trapped in ice cores. These reveal that the world has not been as warm as it is now for a millennium or more. The three warmest years on record have all occurred since 1998; 19 of the warmest 20 since 1980. And Earth has probably never warmed as fast as in the past 30 years - a period when natural influences on global temperatures, such as solar cycles and volcanoes should have cooled us down. Studies of the thermal inertia of the oceans suggest that there is more warming in the pipeline.

Climatologists reporting for the UN Intergovernmental Panel on Climate Change (IPCC) say we are seeing global warming caused by human activities and there are growing fears of feedbacks that will accelerate this warming.