Temperature rise in North America due to 'human activity'

North America is already being affected by man-made global warming, a new study by a team of international scientists has claimed.

The scientists, who include Dr Peter Stott of the Met Office in the UK, said that "human activity is to blame" for an increase in North American temperatures during the 20th century. The study, the first to consider North American warming using data from multiple models, is published today in the journal Science.

Professor David Karoly of the University of Oklahoma led the study, which showed that North American warming, including a 0.8 deg C rise since 1970, was being caused mainly by increasing greenhouse gas concentrations due to human activity. The Met Office has said that global average temperatures have risen by about 0.6-0.7 deg C over the last 100 years.

The team examined five of the world's leading climate models and found that when they included the most important human factors believed to affect climate – including increasing greenhouse gases – they "accurately simulate" the warming observed in North America in the second half of the 20th century.

The models suggested that natural factors alone, including changes in output from the sun, could not explain the observed warming.

The new research shows that human-induced climate change was associated with greater warming in the north than the south of the continent; more warming over land than the surrounding ocean and a faster increase in night-time minimum temperatures than daytime maximum temperatures.

Dr Peter Stott said: "All five climate models agree in showing that the warming observed in North America in recent decades can only be explained as a result of increasing greenhouse gas emissions.

"This adds to the growing evidence that human activity is changing the climate, not just globally, but in specific regions of the world such as North America."

(gmcg)