Earth is kept at a livable temperature by the greenhouse effect. Most incoming shortwave solar radiation passes through the atmosphere, though about one-third is reflected back to space. The surface absorbs about half of this radiation and warms the surface. To balance the energy that is absorbed by the earth, the rest is re-emitted as infrared longwave radiation or reflected back to the atmosphere. Most of this radiation is absorbed by greenhouse gases and re-emitted back to the surface. This is known as the greenhouse effect (Figure 1). Greenhouse gases (carbon dioxide, methane, nitrous oxide and water vapor) are naturally occurring in the Earth’s atmosphere. Without the greenhouse effect, Earth would not be habitable.
A good summary of the greenhouse effect and its importance can be found via the IPCC Frequently Asked Questions: What is the Greenhouse Effect?
Global temperatures are projected to continue to increase through the 21st century. Precipitation is more variable than temperature and may increase in some areas and decrease in others. The range of projections are large; they depend on greenhouse gas emissions and the climate sensitivity. More about climate projections and how they are developed can be found on our Climate Modeling page.
Key Findings of the IPCC reports:
The observed changes in global climate include (Figure 2):
- Increase in average global air and ocean temperatures
- Rise in sea level
- Increased melting of ice and snow
The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) was based on thousands of papers in peer-reviewed journals. This assessment report reported that:
- Warming of the Earth is unequivocal, and
- The warming is very likely due to human activity, with a 90% level of confidence
Human-caused climate change is caused by addition of anthropogenic greenhouse gases to the atmosphere. The addition of these extra greenhouse gases act to exacerbate the natural greenhouse effect. These contributions are predominately from the burning of fossil fuels, but a small amount are also from land use changes. Carbon dioxide emissions from the burning of fossil fuels are the most significant source of anthropogenic greenhouse gases. Global atmospheric concentrations of carbon dioxide have risen almost 100 ppm since their pre-industrial (1750) value of 280 ppm. As of 2010, seasonally adjusted atmospheric concentration of carbon dioxide is 388 ppm. Based on information gathered from ice core data, this value has not been exceeded over the past 650,000 years.
Natural factors, which include solar variation and volcanic activity, are also a driver of climate change. Though these factors certainly influence climate, they do not explain all of the observed warming of the past few decades.
The IPCC’s Summary for Policymakers is a readable, 18 page summary detailing findings regarding the state of observed and projected climate science found in the larger IPCC assessment report.
The United States Global Change Research Program published a recent assessment, ”Global Climate Change Impacts in the United States,” with a good summary of observed and projected global climate changes.