Michael Le Page, biology features editor
"Snow chaos: and still they claim it's global warming." "Snowmageddon delivers another blow to global warming." "The mini ice age starts here."
A few months ago, these were the kind of headlines that were appearing in newspapers and blogs. After very cold winter weather in many parts of the northern hemisphere, the notion of global warming was ripe for mockery. The family of senator Jim Inhofe - who called global warming "a hoax", built an igloo in Washington DC, with a sign saying "Al Gore's new home".
And now? The winter weather has given way to a series of extraordinary heatwaves.
According to meteorologist Jeff Masters, nine countries have recorded their hottest ever temperatures this year, from the 53.5 °C recorded on 26 May in Pakistan to the 44 °C recorded in Russia on 11 July.
If these records are officially confirmed, it will mean more national heat records been set in one year than ever before. So should "global warmists" be crowing about how this record-breaking heat proves they were right all along?
No: the record-breaking heat does not "prove" global warming.
Just as extreme winter weather does not prove the world is cooling, so a few heatwaves do not prove the world is warming. After all, it's not hot everywhere: the southern cone of South America is currently enduring a cold snap that has killed dozens of people.
The various measures of average global temperatures, however, do suggest that surface temperatures are the hottest they have been since records began.
According to the US National Oceanic and Atmospheric Administration, for instance, June was the fourth consecutive warmest month on record. According to NASA, the average temperature over the past 12 months has been the hottest ever.
Hot weather can often be blamed on El Niño. During El Niño events, some of the vast amount of heat stored in the Pacific Ocean is transferred to the atmosphere, so the highest surface temperatures usually occur during El Niños.
But the latest El Niño was not as especially strong. What's more, we are currently getting less heat from the sun than we have for decades. This means that it's the combination of a strong underlying global warming trend due to rising carbon dioxide, together with a moderate El Niño, that explains why the planet is so hot at the moment.
A La Niña has now begun - the opposite of El Niño. In a La Niña the Pacific Ocean soaks up atmospheric heat, lowering surface temperatures. If it's a strong La Niña, 2010 might not turn out to be the hottest calendar year on record. But the next time there is an El Niño, especially if it coincides with a high in solar activity, we are likely to see a lot more records shattered.