When it comes to clarifying the causes of climate change it appears that the situation might not be, as some lobby groups would have us believe, hopeless. At least this is what Simon Tett from the Meteorological Office Bracknell, UK and colleagues report in the 10 June issue of Nature.

They assume that each of a whole host of influences – 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 this century's erratic temperature rise. They then set out to see what proportion of each need be included in their global climate models to generate a temperature record like that observed.

This requires a huge amount of computation. First, they ran the model without any of the so-called influences or 'forcing factors' for a simulated period of 1700 years, just to see how variable the natural climate system was. Answer: not nearly enough to explain why the world is almost a degree warmer than it was 100 years ago. Then it's a question of finding the combination of different forcings that best matches the observations – rather like trying to reproduce a particular colour by mixing different primary hues

But how much of each should be added to the model? For the human-generated greenhouse gases and sulphate aerosols the changes over the past century can be estimated either from direct measurement records or from calculations of, for example, how much sulphate would have accompanied a given amount of carbon dioxide released from fuel burning. Volcanic emissions over the past century are pretty well known from geological observations. Changes in the Sun's output are a little trickier, but can be deduced for example from historical records of sunspots or from measurements of carbon-14 in tree rings – the amount of carbon-14 in the atmosphere is affected by changes in the solar wind.

The researchers' findings were unambiguous: "the temperature changes over the twentieth century cannot be explained by any combination of natural internal variability and the response to natural forcings alone", they conclude. 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.

Thus the rise in 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 and so "masked" the warming effect during that time. All in all, it seems we can lay to rest the idea that recent climate warming is just a freak of nature.