Causes of Climate Change

The Intergovernmental Panel on Climate Change (IPCC) confirmed in its Third Assessment Report that there was "new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities". Although uncertainties in the process of projecting future trends create wide margins for error in the estimates, the IPCC predicted a rise of 1.4 to 5.8 ˚C in global mean surface temperatures over the next 100 years. The negative impact of warming, even at the lower stage of the range is likely to be dramatic. The consequences on humans will be inevitable. People in some areas may benefit from climate change, but mainly they will struggle to cope. Developing countries will suffer more than others as the lack of resources makes them especially vulnerable to adversities or emergencies on any major scale. Yet people in developing countries have created only a small proportion of greenhouse gas emissions.

The earth must shed energy into space at the same rate at which it absorbs energy from the sun. Solar energy arrives in the form of short wavelength radiation. Some of this radiation is reflected away by the earth's surface and atmosphere. Most of it, however, passes straight through the atmosphere to warm the earth's surface. The earth gets rid of this energy (sends it back out into space) in the form of long wavelength, infra-red radiation.

Most of the infra-red radiation emitted upwards by the earth's surface is absorbed in the atmosphere by water vapor, carbon dioxide, and the other naturally occurring "greenhouse gases". These gases prevent energy from passing directly from the surface out into space. Instead, many interacting processes (including radiation, air currents, evaporation, cloud-formation, and rainfall) transport the energy high into the atmosphere. From there it can radiate into space. This slower, more indirect process is fortunate for us, because if the surface of the earth could radiate energy into space unhindered, the earth would be a cold, lifeless place, a bleak and barren planet rather like Mars.

By increasing the atmosphere's ability to absorb infra-red energy, our greenhouse gas emissions are disturbing the way the climate maintains this balance between incoming and outgoing energy.

We have changed, and are continuing to change, the balance of gases that form the atmosphere. This is especially true of such key "greenhouse gases" as carbon dioxide (CO₂ ), methane (CH₄), and nitrous oxide (N₂O) and three industrial gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6). Water vapor is the most important greenhouse gas, but human activities do not affect it directly. Greenhouse gases act like a blanket around the earth. Without this natural blanket the earth's surface would be some 30°C colder than it is today.

The Earth's climate has always changed and evolved. Some of these changes have been due to natural causes, but others can be attributed to human activities such as deforestation and to atmospheric emissions, e.g. from industry and transport, which have led to gases and aerosols being stored in the atmosphere. They are known as greenhouse gases (GHG-s) because they trap heat and raise air temperatures near the ground, acting like a greenhouse on the surface of the planet.

Carbon dioxide is the most important anthropogenic greenhouse gas. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm  (ppm (parts per million) or is the ratio of the number of greenhouse gas molecules to the total number of molecules of dry air. For example: 300 ppm means 300 molecules of a greenhouse gas per million molecules of dry air) in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores. The annual carbon dioxide concentration growth-rate was larger during the last 10 years (1995 - 2005 average: 1.9 ppm per year), than it has been since the beginning of continuous direct atmospheric measurements (1960 - 2005 average: 1.4 ppm per year) although there is year-to-year variability in growth rates. The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land use change providing another significant but smaller contribution.

The global atmospheric concentration of methane has increased from a pre-industrial value of about 715 ppb to 1732 ppb in the early 1990s, and is 1774 ppb in 2005 ppb The atmospheric concentration of methane in 2005 exceeds by far the natural range of the last 650,000 years (320 to 790 ppb) as determined from ice cores. Growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. It is very likely that the observed increase in methane concentration is due to anthropogenic activities, predominantly agriculture and fossil fuel use, but relative contributions from different source types are not well determined.

The global atmospheric nitrous oxide concentration increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005. The growth rate has been approximately constant since 1980. More than a third of all nitrous oxide emissions are anthropogenic and are primarily due to agriculture.

Eleven of the last twelve years (1995 -2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). The updated 100-year linear trend (1906-2005) of 0.74 [0.56 to 0.92]°C is therefore larger than the corresponding trend for 1901-2000 given in the TAR of 0.6 [0.4 to 0.8]°C. The linear warming trend over the last 50 years (0.13 [0.10 to 0.16]°C per decade) is nearly twice that for the last 100 years. The total temperature increase from 1850 - 1899 to 2001 - 2005 is 0.76 [0.57 to 0.95]°C.

Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise (ice caps do not include contributions from the Greenland and Antarctic ice sheets).

Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 [2.4 to 3.8] mm per year.

Average Arctic temperatures increased at almost twice the global average rate in the past 100 years. Arctic temperatures have high decadal variability, and a warm period was also observed from 1925 to 1945.  Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic (by up to 3°C). The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900, with a decrease in spring of up to 15%.

Long-term trends from 1900 to 2005 have been observed in precipitation amount over many large regions. Significantly increased precipitation has been observed in eastern parts of North and South America, northern Europe and northern and central Asia. Drying has been observed in the Sahel, the Mediterranean, southern Africa and parts of southern Asia. Precipitation is highly variable spatially and temporally, and data are limited in some regions. Long-term trends have not been observed for the other large regions assessed11.

Changes in precipitation and evaporation over the oceans are suggested by freshening of mid and high latitude waters together with increased salinity in low latitude waters. {5.2}

Mid-latitude westerly winds have strengthened in both hemispheres since the 1960s.

More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation have contributed to changes in drought. Changes in sea surface temperatures (SST), wind patterns, and decreased snow pack and snow cover have also been linked to droughts.

The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapor.

Widespread changes in extreme temperatures have been observed over the last 50 years. Cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.

There is observational evidence for an increase of intense tropical cyclone (Tropical cyclones include hurricanes and typhoons) activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater.

Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years.