"The greenhouse effect has always been there - the atmosphere that surrounds us traps some of the short wave radiation from the sun: if no atmosphere, life on Earth would not be possible," he says.
The problem is when the changing chemical composition of the atmosphere starts trapping too much heat, and the planet starts heating up.
"The greenhouse effect is increasing - and it's due to man made pollutants, mainly CO2, but also methane, and the CFCs that also cause stratospheric zone depletion."
More CO2 and higher temperatures will produce hundreds of side effects, called feedbacks. Some will tend to correct the greenhouse effect: Plants will grow better, meaning that there will be an increase in oygen and a decrease in CO2. As the Earth gets warmer, there will be more evaporation, more moisture and so more clouds, which will reduce the amount of solar energy reaching the Earth.
Other feedbacks are grimmer: The melting polar ice caps would meanthe surface of the Earth would be less refelctive and so would absorb even more heat from the sun.
Weatherpeople use computer models to try to calculate the net effect of the feedback.
"Ten or fifteen years ago, when modelling first started, they were predicting increases of about five percent in 100 years. That type of change has never been heard of before. Models now are more sophisticated and the estimates are more modest," says McWilliams.
"The consensus is that by the year 2100 - the average world temperature will rise between 1.5 and 3 percent. That's of the order of magnitude of the temperature change in the past 10,000 years - but crammed into 100 years."
But he warns that it's still inexact and that regional variations are hard to predict.
"There could be greater than average increases in some areas."
Take two scenarios for what may happen in Ireland.
The optimistic one is that climate change will be smooth: "There might be a gradual shift northwards of the lines of longtiutude and Ireland would end up with a warmer climate."
On the other hand, if the average temperature over Atlantic were to increase, it would start melting the ice in the Artic. The resulting stream of fresh water could block the warm Gulf Stream, which is what keeps Ireland relatively warm. If the Gulf Stream was blocked, Ireland's climate could become like the Baltic.
Is there evidence of other climate change resulting from global warming?
"Unusual weather that has been reported is generally regarde as being within the bounds of normal variations in the weather," he says.
"But it's a very difficult question - scientists are confident that the average temperature is rising. Most are reasonably sure that CO2 increases are a big factor in this. It's a big jump from that to saying that this change is already affecting the weather. I don't think they've made that jump yet." "If global warming continues it will almost certainly have an effect on the weather," he warns.
"Last year there were 19 hurricanes over the Atlantic, the most since 1933. It's not inconsistent with climate change. But neither, obviously, is it outside the normal range of climactic variability."
So what can be done about it?
"It's a very slow process to reverse it. The CO2 levels might gradually return to normal, but if it's a positive feedback depends on the feedback that wins out. If it's a positive feedback, even if we reduce CO2 emmissions to zero, global warming might still accelerate."
"CO2 power generating stations, factories, the use of fossil fuels in any form emits CO2. Global CO2 emmissions have to fall - but nobody wants to do it, because they think it will affect their prosperity. Alternative fuels are the only answer."
There is quite an active anti global warming lobby in the US in particular - their clients include prominent oil and coal interests and other major conglomerates. The International Herald Tribune recently carried a piece from one such luminary who said the global warming theory was totally bogus and that the climate change treaties should be scrapped.
There are suspicions that they have ties to industries that would be hard hit by CO2 reductions.
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