The pollution accumulates, accumulates, accumulates, until a cold front comes in and the winds sweep it away.”Mickley and colleagues used a computer model, an approach commonly used by climate scientists to predict weather and climate changes.She told a meeting of the American Association for the Advancement of Science that the model predicted a 20 per cent decline in summer cold fronts out of Canada.“If this model is correct, global warming would cause an increase in difficult days for those affected by ozone pollution, such as people suffering with respiratory illnesses like asthma and those doing physical labour or exercising outdoors,” she said.Earlier this week 141 nations signed the UN Kyoto Protocol aimed at cutting greenhouse gas emissions that fuel global warming. The US, which produces the most pollution of any country, has refused to sign it.