Today, the world is warmer than it has been since we started making reasonably accurate measurements about 150 years ago. It has been warming since Victorian times, off and on, with a few periods of no change. Over this time, it has become about 0.7 degrees C warmer. The Intergovernmental Panel on Climate Change (IPCC) says that mankind's influence on our climate became apparent around 1960 - and that, since 1980, the global temperature has risen by 0.4 degrees C; mostly due to mankind.
While we do live in the warmest decade since proper measurements began our temperatures do not seem to be unprecedented. There is good evidence that 1,000 years ago the medieval climatic optimum was just as warm, if not warmer. Before that the Roman warm period, and the Bronze Age warm period might have been just as hot. What is different today is that as a result of industrialisation, mankind is pumping 90 million tonnes of greenhouse gasses into the atmosphere every 24 hours, but calculating the effect of it is difficult.
Despite what you might read, forecasting the extent of global warming in the future is an uncertain task. Greenhouse gasses trap more heat from the sun, so the world will warm. But by how much is a matter of speculation. This is a mainstream scientific issue, not an extreme position. Climatologists are uncertain about what they call the earth's sensitivity to greenhouse gasses. Do they have a big effect, or a small one? And what is the contribution made by so-called feedback mechanisms in our climatic system - do they make the situation better or worse?
Then there is the interplay of short-term natural climatic cycles called decadal variations - that are atmospheric, oceanic and solar cycles. Finally there are longer, millennial effects to account for as well. Climate scientists take a long view of things. Thirty years is enough, they say, to get away from the effects of weather variations and see what long term changes are happening. Over the past 30 years, the world has warmed. But it has not warmed at all since 2001. Although only a decade in length, scientists are puzzled why - as greenhouse gasses increase - the temperature has not. It may be a temporary blip, or possibly an indication that we do not fully understand what is going on.
The sun is, by far, the biggest energy source for our climate and scientists know that changes in its activity can cause climatic effects. Such was the case in the 17th century when a prolonged period of no sunspots corresponded to a cold spell called the Little Ice Age. It was the same at the beginning of the 19th century.
Recently, after two hundred years of increasing activity, our sun has once again gone into a quiet spell. It might mean that it has a cooling effect on the earth in decades to come. Predictions from climate models suggest that man-made warming will be much larger though. That is, of course, if the climate computer models are correct. For a subject that is often dealt with using slogans, pamphlets and newspaper headlines - climate change is actually a complex issue.
The earth's climate system involves the absorption of over a hundred petawatts - that is more than a hundred million times the energy of a nuclear power station - of energy from the Sun. It goes into the atmosphere, oceans and land. The atmosphere and oceans are moving, constantly exchanging energy and the atmosphere moves over a variable land surface heating and cooling as it does.
Never before have scientists attempted to simulate such a complex system as the earth's climate in a computer. What has emerged, an ever-warming world, might be right. Or it may not. If the current temperature standstill does not change soon, nature may be telling us that we have got it wrong.