News that carbon dioxide levels have reached 400 parts per million – a level widely thought by scientists not seen for at least 3 million years – caught the headlines last month.
It comes at a time when global temperatures have remained largely flat since the late 1990s, despite climate forecasts of a rise of 0.2C per decade.
Through the 1980s and 1990s, as carbon dioxide levels rose, so did global temperatures, until a peak in 1998.
Since then it’s become increasingly clear that the science is not as straightforward as it seemed, and after 15 years of flat-lining temperatures, scientists are trying to work out what has caused this apparent halt in global warming.
One of the criticisms of mainstream climate scientists, by sceptics, is that they have been very slow in acknowledging this obvious levelling-off in global temperatures.
So why have we seen very little warming in recent years? Is the science in which global warming projections are based flawed?
Calculations based on simple physics show that if levels of carbon dioxide double, the earth’s temperature should rise by approximately 1C.
Mainstream climate scientists believe that this initial rise will then lead to further global warming due to feedback mechanisms.
For example, melting ice and permafrost could potentially release billions of cubic feet of natural methane gas, which is around 20 times more powerful a greenhouse gas than carbon dioxide.
Levels of water vapour in the atmosphere are likely to increase, itself a much more powerful greenhouse gas than carbon dioxide.
Some climate sceptics argue that a warmer earth with more water vapour in its atmosphere will lead to a cloudier planet; more cloud cover will act to cool the planet by reflecting some of the suns warmth back onto Space; the earth is in equilibrium with itself, and will adjust itself accordingly, according to this theory.
Climate sceptics, though, have yet to come forward with a plausible reason as to why carbon dioxide wouldn’t increase the earth’s temperature; in other words, why in this instance would basic physical laws not apply?
But assuming that the science is sound, an explanation is still required as to why the rise in global temperatures has ground to a halt and why it wasn’t forecast.
Because according to data used by Climate lab book in association with the National Centre for Atmospheric Science (based at Leeds University) ‘global temperatures are tracking at the bottom of the range in which 90 per cent of the model simulations lie’.
Put simply, the vast majority of climate models predicted that global temperatures would be higher than they are today.
So why are global temperatures not higher?
One explanation put forward lies in the oceans, which cover 70% of the earth’s surface.
The top two and a half metres of the ocean can hold as much heat as the atmosphere. So could the missing global warmth be here?
Some scientists believe it's buried deep in the oceans, but if this is the case, measuring it is a real problem.
Some recent research has suggested that the earth may not be quite as sensitive to increased levels of carbon dioxide as scientists previously thought.
But whatever the explanation as to where the ‘missing heat’ is, and whatever the actual level of warming turns about to be, with carbon dioxide levels continuing to rise year on year, altering the chemical composition of the atmosphere, most scientists believe that further global warming is only a matter of time, but by how much and how fast is the real conundrum.
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