Climate change caused by human activity has been detected on a local level, an international group of scientists says.
North America is hotting up
The researchers compared temperatures in North America between 1900 and 1999 with what one might expect if man had - and had not - had an influence.
And in the last 50 years, the rise in temperature is just what one would predict if man-made greenhouse gases were having an impact, they claim.
The research is published in this week's edition of the journal Science.
It is getting warmer in North America. Since 1900, the average temperature has risen by one degree Celsius, and 80% of that increase has happened since 1970.
What is not certain, is why this has happened. Some blame the greenhouse gases produced by humans; others say it is all part of a natural fluctuation.
Peter Stott from the Met Office in the UK, and his colleagues, used sophisticated computer models to address the question.
They created several virtual 20th Century North Americas, complete with sea, trees, mountains, volcanoes, clouds and weather systems. The model North Americas were capable of accurately mimicking the climate of the real-life version.
Then the researchers introduced different influences to each model, to see how they affected the climate.
To some, they introduced lots of volcanic action and solar activity, which can cause natural climate change. And to others, they introduced greenhouse gases, ozone and sulphate aerosols - all of which contribute to man-made climate change.
Each of the models predicted what the climate of North America should have looked like, under a particular set of influences.
Then they looked at what really happened to the climate of North America over the past century, and measured how well the models matched up.
The idea was that if a particular model of North America closely matched the real thing, one could assume it did so because the influences at play in the real and virtual world were the same.
In the second half of the century - 1950-1999 - the climate change of North America closely matched the "human influence" model. But in the first half of the century, the climate change matched the "natural influence" model.
So the models were suggesting that North America warmed up for natural reasons during the first half of the century but, in the second half, man caused the climate change.
"The model that included greenhouse gases and sulphate aerosols agreed with what we really saw in the second half of the century," said Dr Stott. "But when we looked at the models that didn't include these things, they didn't fit."
The researchers also compared the air temperature above the land with that on the surface of the ocean. They found that the air above the land was heating up much quicker.
This is inconsistent with what we might expect if we were witnessing a natural fluctuation, the researchers say, because they are usually characterised by a rise in ocean temperature.
"The fact the air is warming faster on land than on the ocean surface bares the human finger-print," Dr Stott told BBC News Online. "That is because normal fluctuations are often caused by changes in the ocean systems."
Previous studies on the possible causes of 20th Century warming have generally concentrated on global-scale patterns of climate change.
"There is a whole body of evidence for global climate change," said Dr Stott, "but now we have found it locally."