Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. The changes observed over the last several decades are most likely due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.
Because there is considerable uncertainty in current understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases and aerosols, current estimates of the magnitude of future warming should be regarded as tentative and subject to future adjustments (either upward or downward).
Greenhouse gas sources and sinks
Most greenhouse gases have both natural and human-made emission sources. There are, however, significant natural mechanisms (land-based or ocean-based sinks) for removing them from the atmosphere. Increased levels of anthropogenic emissions have pushed the total level of greenhouse gas emissions (both natural and anthropogenic) above the natural absorption rates for these gases. This positive imbalance between emissions and absorption has resulted in the continuing growth in atmospheric concentrations of these gases.
Water vapour is the most common greenhouse gas present in the atmosphere. It is emitted into the atmosphere in enormous volumes through natural evaporation from oceans, lakes and soils and is returned to Earth in the form of rain and snow. The recent IPCC report, however, cites a possible positive feedback from increased water vapour formation due to increased warming caused by rising atmospheric CO2 concentrations. Elevated atmospheric temperatures increase the ‘water-holding’ capability of the atmosphere. According to some of the IPCC emission scenarios, higher water vapour content could double the predicted atmospheric warming above what it would be if water vapour concentration stayed constant. These scenarios have an element of uncertainty due to the possible countervailing effect of increased cloud formation, which can act to cool the planet by absorbing and reflecting solar radiation or warm the planet through the emission of long-wave radiation. Increases in atmospheric temperatures would not necessarily result in increased concentrations of water vapour, because most of the atmosphere today is under-saturated.