Climate change is here and we had better act now to curb its impact on our lives and society, writes Prof William Reville.
Nobody disputes that the world is warming but some argue that the warming is natural and not forced by human activities. The latest report by the Intergovernmental Panel on Climate Control (IPCC), issued last February, removes any lingering doubt on the latter point.
The IPCC reports that it is "very likely" (ie the probability is more than 90 per cent), that global warming is caused by human activities, primarily from the burning of fossil fuel. We can still do something about the longer-term effects of this problem, but, we had better get cracking on this right away or we will bequeath an awful problem to our children. A clear summary of the IPCC report appears in the August 2007 issue of Scientific American. All IPCC reports and summaries are available at www.ipcc.ch.
The current report is the fourth of a series (1990, 1995, 2001 and 2007) issued since the IPCC was formed in 1988 to assess scientific information on climate change. Lead authors are experts and are nominated by governments. The IPCC assessments are reviewed in detail by the broader scientific community before publication, thereby ensuring the highest scientific credibility.
The evidence behind the IPCC reports is overwhelming. First of all, the world is certainly warming. The 2001 IPCC report estimated a warming trend of 0.60 +/- 0.20 degrees over the period 1901-2000. The latest report updates the estimate to 0.74 +/- 0.18 degrees for the period 1906-2005. Also the 1956-2005 trend is 0.65 +/- 0.15 degrees, showing that the bulk of 20th-century warming occurred in the last 50 years. It is also reported that 11 of the past 12 years are the warmest since reliable records began in 1850. The odds of such warm sequential years happening by chance are miniscule.
The IPCC reviewed all studies of changes in positive and negative radiative forces (influences that cause the world's climate to grow warmer or colder) over the past 200 years. The greatest positive forces by far are the long-lived greenhouse gases carbon dioxide, methane, nitrous oxide and halocarbons, and, of these, carbon dioxide is easily the biggest contributor.
Concentrations of carbon dioxide, methane and nitrous oxide remained stable in earth's atmosphere for 19,000 years, but started to increase abruptly about 200 years ago.
Today, carbon dioxide levels are 35 per cent greater than pre-industrial levels, methane levels are 2.5 times pre-industrial levels and nitrous oxide levels are 20 per cent higher. Halocarbons gases, used as refrigerants and gas propellants, have no natural source. Increased levels of carbon dioxide and nitrous oxide are largely due to the burning of fossil fuels and the increased concentration of methane is due to agricultural activity.
Human activities other than greenhouse gas emissions affect the radiative forces acting on the earth. Two in particular have a cooling effect - surface albedo (reflectivity of the earth) and aerosols. A lighter surface, such as snow, reflects more sunlight than a darker surface. Changed land use over the last century had a cooling albedo effect. Aerosols are airborne particles produced both by natural (eg dust storms, volcanoes) and human sources (burning of fossil fuels). Aerosols reflect sunlight and also seed cooling clouds. Changes in solar activity can also naturally warm or cool the earth and this natural solar forcing has had a small warming effect on the planet.
When all the warming and cooling forces that operated on earth since pre-industrial times are added up, the warming effects overwhelm the cooling effects. About 90 per cent of the warming is due to human emissions of greenhouse gases and about 10 per cent of the warming is due to changes in solar activity.
The warming world shows many symptoms - sea level rise (3.1 +/- 0.7mm/year since 1978), reduction of Arctic Sea ice (2.7 per cent +/- 0.6 per cent per decade reduction since 1978), decreased spring snow cover in northern mid-latitudes, shrunken glaciers and ice-sheets, and so on.
Rainfall has increased in several large regions, including northern Europe and has decreased in other regions, for example across South Africa.
The extent of future global warming depends on what we do now about greenhouse gas emissions. But, even if we entirely stopped emitting these gases now, the world would still continue to warm somewhat. This is because it takes a long time for the gases we have already emitted to mix completely in the atmosphere and for the warmed surface layers of the oceans to transfer some of the heat to the ocean depths.
The IPCC models predict, for a range of plausible emissions over the next 20 years, an average continuing warming of 0.2 degrees per decade. About half of this represents inertia in the climate system responding to the current level of greenhouse gases. Longer term calculations over the 21st century depend on emissions and predict a further warming of anywhere between 1.8 degrees and 4 degrees. The corresponding range of sea level rise is 30 to 40cm and possibly up to 60cm if ice-sheet loss accelerates.
The most probable consequences in Europe are inland flash floods; health threatening heat waves in the south plus wildfires, water shortages and endangered crop production; reduced summer rainfall; influx of new infectious diseases such as malaria; widespread flooding of low-lying areas and some or all of these arriving by 2080.
If temperature increase is not slowed, it will overwhelm us. At present the concentration of carbon dioxide equivalents in the atmosphere is about 400 parts per million. To keep future temperature increases to the lowest of the projections given above, IPCC estimates that the world must stabilise atmospheric greenhouse gases at 445ppm by 2015. Whatever needs doing to effect this must be done.
• Prof William Revilleis Associate Professor of Biochemistry and Public Awareness of Science Officer at UCC - http://understandingscience.ucc.ie