Global warming caused by man, report shows

The rise in global temperature of about a quarter of a degree since the 1940s seems to be due mainly to increases in greenhouse gases, although concomitant increases in sulphate aerosols offset this up to the mid-1970s, the research carried out by the Meteorological Office Bracknell, UK says.

“The temperature changes over the twentieth century cannot be explained by any combination of natural internal variability and the response to natural forcings alone,” the report concludes. Rather, it seems necessary to include some human-induced component in the climate forcing throughout the century, although natural variability and changes in the Sun’s output could also have made significant contributions.

Simon Tett and colleagues at the Meteorological Office assumed that a number of influences or ‘forcing factors’ – natural climate variability, changes in solar activity, volcanic emissions and changes in amounts of human-induced greenhouse gases and sulphate aerosols (particles that result from the burning of fossil fuels) – might play a part in causing the temperature rises of the last 100 years.

The researchers then set out to see what proportion of each influence needed to be included in their global climate models to generate a temperature record like that observed.

The researchers ran the model without any of the forcing factors for a simulated period of 1700 years, to see how variable the natural climate system was. They found the influences were not nearly enough to explain why the world is almost a degree warmer than it was 100 years ago.

The next step was to find the combination of different influences that best matches the observations.

For the human-generated greenhouse gases and sulphate aerosols, the changes over the past century were estimated either from direct measurement records or from calculations of how much sulphate would have accompanied a given amount of carbon dioxide released from fuel burning. Volcanic emissions over the past century were calculated from geological observations. Changes in the Sun’s output were deduced from historical records of sunspots or from measurements of carbon-14 in tree rings as this is affected by changes in the solar wind.