WCMC Climate Change

The Changing Climate
The world's climate has always been highly variable. Many changes are driven by natural factors, but others are now the result of human activities.

Human influences on the climate

•The use of fossil fuel currently accounts for 80 to 85% of the carbon dioxide being added to the atmosphere. Carbon dioxide is an important greenhouse gas and increasing concentrations lead to a heating of the atmosphere. Concentrations have increased by 25% over the last 200 years, primarily as a result of burning coal, oil, and natural gas (e.g., in automobiles, industry, and electricity generation).
• Land use changes, e.g., clearing land for logging, ranching, and agriculture, account for a further 15 to 20% of current carbon dioxide emissions. Vegetation contains carbon that is released as carbon dioxide when the vegetation decays or burns.
• The amount of carbon dioxide in the atmosphere will double during the twenty-first century if the current trends in emissions continue, with further increases thereafter. The amounts of several other greenhouse gases will increase substantially as well.

Natural changes in climate

Large volcanic eruptions put tiny particles in the atmosphere that block sunlight, resulting in a surface cooling of a few years’ duration.
Variations in ocean currents change the distribution of heat and precipitation. El Niño events (periodic warming of the ocean surface central and eastern tropical Pacific Ocean) typically last one to two years and change weather patterns around the world, causing heavy rains in some places and droughts in others.
Over longer time spans, tens or hundreds of thousands of years, natural changes in the energy received from the sun, or in the balance of greenhouse gases or dust in the atmosphere have caused the climate to shift from ice ages to relatively warmer periods, such as the one we are currently experiencing.

Facts and figures

Greenhouse gas is a term used to describe a range of gases which trap radiation emitted from earth’s surface, and hence keep the planet far warmer than it would otherwise be. Carbon dioxide, methane, nitrous oxides and ozone are all greenhouse gases. The order of importance in contributing to human induced global warming is carbon dioxide (70%), methane (20%) and others (10%).
Average global surface temperatures have increased by 0.3-0.6°C since the late nineteenth century, most of this (0.2-0.3°C) has occurred in the last 40 years. Regional analysis of these statistics have shown even greater increases in some areas: Arctic regions have shown a 0.6°C rise since 1979.
There is slightly more carbon dioxide in the northern hemisphere than in the southern hemisphere. The difference arises because most of the human activities that produce carbon dioxide are in the north and it takes about a year for northern hemispheric emissions to circulate through the atmosphere and reach southern latitudes.
The global average sea level has risen by 10 to 25 cm over the past 100 years. It is likely that much of this rise is related to an increase of 0.30.6 C in the lower atmosphere’s global average temperature since 1860.
Glaciers are in retreat on every continent. This loss of ice over the past 100 years has added about 2 to 4 cm to the sea level.
Temperature has not increased as much as expected from the observed CO₂ increase. This is related to increases in the amounts of tiny particles in the air arising, for instance, from industrial activities or volcanic eruptions. These block out some sunlight and induce a reversing cooling effect.

Future scenarios

Increases of 1 to 3.5°C (about 2 to 6°F) in globally averaged surface temperatures have been projected by the Intergovernmental Panel on Climate Change (IPCC) by the year 2100, as compared with 1990. This projection is based on estimates of future concentrations of greenhouse gases and sulphate particles in the atmosphere. The average rate of warming of the Earth’s surface over the next hundred years will probably be greater than any that has occurred in the last 10,000 years, the period over which civilisation developed. However, specific temperature changes will vary considerably from region to region.
The maximum warming is expected to occur in the Arctic in winter.
Both evaporation and precipitation will increase in many regions, according to most climate change models, as will the frequency of intense rainfalls. Some regions that are already drought-prone may suffer longer and more severe dry spells. In spring, faster snow melt may aggravate flooding.
Sea levels will rise another 15 to 95 cm by the year 2100 (with a "best estimate" of 50 cm). This will occur from the combination of thermal expansion of ocean water with the increased influx of freshwater from melting glaciers and ice-sheets. The projected rise is two to five times faster than the rise experienced over the past 100 years.
• Extreme weather events such as tropical storms are more difficult to predict, as indeed are the more fine-scale patterns of change at local levels. This uncertainty results from the existence of large natural regional variations, as well as limitations in computer models and the understanding of the relationship between local and global climate.
Changes may be very rapid. Most predictions are based on the assumption that the global climate will change gradually. However, there is evidence of relatively abrupt changes to climate at various phases in the Earth’s history. Similar changes may well be induced again in the near future. It has been suggested, for example, that increasing temperatures may result in massive release of carbon dioxide from the soil as permafrost melts and peat deposits are broken down. This will add to climate warming and further accelerate permafrost melting. Other models have raised the possibility of abrupt changes in ocean currents.