Climate Change > Warming in progress

Warming in progress

The full instrumental temperature record of the last 150 years. Source: www.globalwarmingart.com

There is wide agreement among climate researchers - and now also among policy makers - that man-made emissions of greenhouse gases are affecting the Earth's radiation balance.

It will get warmer, but it is still difficult to say how big the effect will be and where it will be greatest.

The scientific aspects of the climate issue have been described in four major assessment reports from IPCC, the Intergovernmental Panel on Climate Change.

In the third report (2001), the panel said it was “likely” that human activities lay behind the trends observed in various parts of the planet. “Likely” in IPCC terminology means between 66 and 90 per cent probability.

In the fourth report (2007) the panel concludes, it is at least 90 per cent certain (“very likely”) that human emissions of greenhouse gases, rather than natural variations, are warming the planet’s surface.

The IPCC has this to say about developments to date:

• The mean global temperature has risen by 0.74°C over the last 100 years (1906–2005). Over the last fifty years the rate of increase has risen to around 0.13°C per decade. Average Arctic temperatures increased at almost twice the global average rate in the past 100 years.

• The biggest single impact on the climate that results from human activities comes from emissions of carbon dioxide. This is produced by burning fossil fuels, particularly coal and oil, but also through changes in land use. The level of carbon dioxide in the atmosphere has risen by over 35 per cent since the mid-nineteenth century (from around 280 ppm in 1850 to 379 ppm in 2005).

• Levels of other greenhouse gases (methane and nitrous oxide) have also risen at the same time as a result of human activities, especially agriculture.

• The combined radiative forcing of the increases in carbon dioxide, methane and nitrous oxide is +2.30 W/m2, and its rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years. Other gases that contribute to global warming include ozone (+0.35) and halocarbons (+0.34).

• There are also human-produced emissions into the atmosphere that lower the surface temperature of the Earth and so counteract some of the warming, such as sulphur particles, which are produced by burning coal and oil. These are estimated to produce an overall cooling effect equivalent to -1.2 W/m2. Although more reliable background data now exists, there is still some uncertainty about the precise climate effects of these particles.

• The combined effect of the rise in greenhouse gas and particle levels and change in surface albedo between 1750 and 2005 is an additional net heat input of +1.6 W/m2 (+0.6 to +2.4) at the surface.

• Between 1961 and 2003 the oceans of the world have risen by almost eight centimetres. This rise is partly due to seawater expanding as a result of the warming of the oceans (down to a depth of 3,000 metres) and partly due to the melting of glaciers.

• Over the period 1993–2003 the rate at which the seas are rising has increased to around twice the rate for the previous forty years. This increase is due mainly to increasing expansion of seawater as a result of warming.

• Long-term trends from 1900 to 2005 have been observed in the amounts of precipitation 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.

• Some extreme weather events have become more frequent, while others have become less frequent. Examples include a fall in the number of cold winter nights and days with frost over land areas, while at the same time there have been increases in the number of hot summer days and warm summer nights. Both these trends are probably due to the enhanced greenhouse effect.

• One conclusion that could have clear political repercussions is that it is now considered likely (between 66 and 90 per cent probability) that very severe Atlantic hurricanes have become more frequent. This is a politically sensitive issue in the United States, since hurricanes are a natural phenomenon that generally hit the US hardest of all. The Bush administration has denied the link between emissions and climate change for a long time, although its tone has begun to change recently.

Last modified: 21 July 2007