The results of a new study by climate scientists in the US are described as “the smoking gun that we were looking for”, removing any doubt that anthropogenic (human induced) activities are the cause of global warming.
The results, just published in the journal Science, base their findings on computer models of climate, and on measurements of temperature in the oceans gleaned from the thousands of scientific buoys and floats which monitor sea temperature.
According to the study, planet Earth is reflecting less energy back into space than it is absorbing from the sun.
The study attempts to calculate the Earth’s “energy imbalance” – the difference between the amount of energy received at the top of the atmosphere from solar radiation, and the amount that is given back into space.
In an interview with BBC News, Gavin Schmidt, one of the research team from the US space agency’s (Nasa) Goddard Institute for Space Studies in New York, said “Measuring the imbalance directly is extremely difficult, because you are looking for a very small number on a background of very large numbers,”
“But we know how much energy is going into the oceans – that has been measured and over the last 10 years confirmed by satellites and in-situ measurements – and from our understanding of atmospheric physics, that has to be equal to the imbalance at the top of the atmosphere.”
The reason the extra energy is trapped, the researchers say, is the human-produced greenhouse effect – elevated levels of gases such as carbon dioxide that absorb radiation from the Earth’s surface which would otherwise disappear into space.
“This is almost unprecedented,” said Gavin Schmidt. “The normal state of the atmosphere is that pretty much the same amount of energy that comes in leaves; and only when there are very large changes is that going to change.
“Historically, those changes have happened very slowly; but what we are doing now is we are changing that imbalance at a rate which appears to be unprecedented over at least a thousand years and possibly longer.”
However, there is a time lag between the absorption of energy at the top of the atmosphere and resultant heating of the oceans. The extra energy already trapped, according to the Nasa team, means that a global temperature rise of a further half a degree Celsius is inevitable, even if human production of greenhouse gases could be turned off tomorrow.
Computer climate models have grown much more sophisticated over the years. But there are still problems modelling some atmospheric processes, notably heat convection within clouds, to the extent that some climate scientists do not accept the study’s findings.
And any model can only be as accurate as the data that goes into it. There is still a need, most researchers agree, for more data from the oceans, and on the role of aerosols (small particles of dust, soot, soil and other substances) in the atmosphere; but gathering that data is easier said than done.
Source: Based on BBC News report