What is climate change?

What is climate change?

The planet's climate has constantly been changing over geological time. The global average temperature today is about 15C, though geological evidence suggests it has been much higher and lower in the past.
However, the current period of warming is occurring more rapidly than many past events. Scientists are concerned that the natural fluctuation, or variability, is being overtaken by a rapid human-induced warming that has serious implications for the stability of the planet's climate.

What is the "greenhouse effect"?

The greenhouse effect refers to the way the Earth's atmosphere traps some of the energy from the Sun. Solar energy radiating back out to space from the Earth's surface is absorbed by atmospheric greenhouse gases and re-emitted in all directions.

The energy that radiates back down to the planet heats both the lower atmosphere and the surface. Without this effect, the Earth would be about 30C colder, making our planet hostile to life

Scientists believe we are adding to the natural greenhouse effect with gases released from industry and agriculture (known as emissions), trapping more energy and increasing the temperature. This is commonly referred to as global warming or climate change.

The most important of these greenhouse gases in terms of its contribution to warming is water vapour, but concentrations show little change and it persists in the atmosphere for only a few days.

On the other hand, carbon dioxide (CO2) persists for much longer (it would take hundreds of years for it to return to pre-industrial levels). In addition, there is only so much CO2 that can be soaked up by natural reservoirs such as the oceans.

Most man-made emissions of CO2 are through the burning of fossil fuels, as well as through cutting down carbon-absorbing forests.

Other greenhouse gases such as methane and nitrous oxide are also released through human activities, but their overall abundance is small compared with carbon dioxide

What is the evidence for warming?

Temperature records going back to the late 19th Century show that the average temperature of the Earth's surface has increased by about 0.8C (1.4F) in the last 100 years. About 0.6C (1.0F) of this warming occurred in the last three decades.

Satellite data shows an average increase in global sea levels of some 3mm per year in recent decades. A large proportion of the change in sea level is accounted for by the thermal expansion of seawater. As seawater warms up, the molecules become less densely packed, causing an increase in the volume of the ocean.

The effects of a changing climate can also be seen in vegetation and land animals. These include earlier flowering and fruiting times for plants and changes in the territories (or ranges) occupied by animals.