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Our planet's climate is anything but simple. All kinds of factors influence it, from massive events on the Sun to the growth of microscopic creatures in the oceans, and there are subtle interactions between many of these factors.

Yet despite all the complexities, a firm and ever-growing body of evidence points to a clear picture: the world is warming, this warming is due to human activity increasing levels of greenhouse gases in the atmosphere, and if emissions continue unabated the warming will too, with increasingly serious consequences.

Yes, there are still big uncertainties in some predictions, but these swing both ways. For example, the response of clouds could slow the warming or speed it up. With so much at stake, it is right that climate science is subjected to the most intense scrutiny. What does not help is for the real issues to be muddied by discredited arguments or wild theories.

So for those who are not sure what to believe, here is our round-up of the 26 most common climate myths and misconceptions.

So what's going on? It is true that human emissions of CO2 are small compared with natural sources. But the fact that CO2 levels have remained steady until very recently shows that natural emissions are usually balanced by natural absorptions. Now slightly more CO2 must be entering the atmosphere than is being soaked up by carbon "sinks".

Similarly, parts of the oceans release about 330 Gt of CO2 per year, depending on temperature and rates of photosynthesis by phytoplankton, but other parts usually soak up just as much – and are now soaking up slightly more.

Ocean sinks
Human emissions of CO2 are now estimated to be 26.4 Gt per year, up from 23.5 Gt in the 1990s, according to an Intergovernmental Panel on Climate Change report in February 2007. Disturbances to the land – through deforestation and agriculture, for instance – also contribute roughly 5.9 Gt per year.

About 40% of the extra CO2 entering the atmosphere due to human activity is being absorbed by natural carbon sinks, mostly by the oceans. The rest is boosting levels of CO2 in the atmosphere.

How can we be sure that human emissions are responsible for the rising CO2 in the atmosphere? There are several lines of evidence. Fossil fuels were formed millions of years ago. They therefore contain virtually no carbon-14, because this unstable carbon isotope, formed when cosmic rays hit the atmosphere, has a half-life of around 6000 years. So a dropping concentration of carbon-14 can be explained by the burning of fossil fuels. Studies of tree rings have shown that the proportion of carbon-14 in the atmosphere dropped by about 2% between 1850 and 1954. After this time, atmospheric nuclear bomb tests wrecked this method by releasing large amounts of carbon-14.

Measurements of CO2 levels over the past 50 years do not show any significant rises after eruptions. Total emissions from volcanoes on land are estimated to average just 0.3 Gt of CO2 each year – about a hundredth of human emissions.

While volcanic emissions are negligible in the short term, over tens of millions of years they do release massive quantities of CO2. But they are balanced by the loss of carbon in ocean sediments subducted under continents through tectonic plate movements. Ultimately, this carbon will be returned to the atmosphere by volcanoes.

But climate change is not an on-off switch. It is a continuing process. The sooner we stabilise atmospheric concentrations of greenhouse gases, the sooner we can reduce our impact on the climate and minimise the risk of reaching tipping points that will make preventing further warming even harder. Even if we only manage to slow warming rather than prevent it, societies will have more time to adjust to the changes.

It is true that the action taken so far, such as the Kyoto Protocol, will only have a marginal effect. The protocol’s authors have always described it as a first step. But even before it came into effect in 2005, the protocol has triggered some profound thinking among governments, corporations and citizens about their carbon footprint and how to reduce it. Industrialised countries such as the UK are planning for emissions reductions of 60% or more by mid-century.

We may find that once the process has begun, the world loses its addiction to carbon fuels surprisingly quickly. Natural scientists fear “tipping points” in the climate system. But there are also tipping points in social, economic and political systems. Once under way, things can happen fast.

Political issue
The great majority of the extra carbon dioxide in the atmosphere was put there by the developed world, with the US alone responsible for an estimated quarter of emissions since 1750. Future emissions may be dominated by large developing countries like China and India. While neither can be blamed for climate change so far, they clearly have to be part of the solution. That is probably the biggest challenge.

But this is primarily a political issue. The industrialised nations have already emitted enough carbon dioxide to trigger significant warming. Humanity cannot afford for the developing world to take the same path. So a deal has to be done to prevent that. But today the technology to develop on a low-carbon path is much further advanced. And costs are coming down fast.

A new deal to save the world from climate change will probably involve large flows of technology and cash to the developing world. There are precedents for this. Developing countries are already being paid in cash and technology for not using ozone-destroying chemicals in refrigerators and air-conditioning systems. The same must be done on a bigger scale to halt climate change.
To repeat, this is not primarily a technological or even an economic problem, as huge as these challenges are. It is a political problem. And in politics, most things can be done if there is the will.

Since 2001, there have been repeated claims that the reconstruction is at best seriously flawed and at worst a fraud, no more than an artefact of the statistical methods used to create it.

Details of the claims and counterclaims involve lengthy and arcane statistical arguments, so let's skip straight to the 2006 report of the US National Academy of Science. The academy was asked by Congress to assess the validity of temperature reconstructions, including the hockey stick.

"Array of evidence"
The report states: "The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on ice caps and the retreat of glaciers around the world".

Most researchers would agree that while the original hockey stick can – and has – been improved in a number of ways, it was not far off the mark. Most later temperature reconstructions fall within the error bars of the original hockey stick. Some show far more variability leading up to the 20th century than the hockey stick, but none suggest that it has been warmer at any time in the past 1000 years than in the last part of the 20th century.

It is true that there are big uncertainties about the accuracy of all past temperature reconstructions, and that these uncertainties have sometimes been ignored or glossed over by those who have presented the hockey stick as evidence for global warming.

Climate scientists, however, are only too aware of the problems and the uncertainties were both highlighted by Mann's original paper and by others at the time it was published.

Similarly, while we cannot predict the weather in a particular place and on a particular day in 100 years time, we can be sure that on average it will be far warmer if greenhouse gases continue to rise.

While weather and to some extent climate are chaotic systems that does not mean that either are entirely unpredictable.

The unpredictable character of chaotic systems arises from their sensitivity to any change in the conditions that control their development. What we call the weather is a highly detailed mix of events that happen in a particular locality on any particular day – rainfall, temperature, humidity and so on – and its development can vary wildly with small changes in a few of these variables.

Climate, however, is the bigger picture of a region's weather: the average, over 30 years (according to the World Meteorological Association's definition), of the weather pattern in a region. While weather changes fast on human timescales, climate changes fairly slowly. Getting reasonably accurate predictions is a matter of choosing the right timescale: days in the case of weather, decades in the case of climate.

The crucial point is that unforced variability occurs within a relatively narrow range. It is constrained by the major factors influencing climate: it might make some winters bit a warmer, for instance, but it cannot make winters warmer than summers.

Put the other way round, two or three warmer winters in a row could be due to unforced variability rather than global warming, just as two or three high scores in pinball do not necessarily mean the table is tilted. But the more warmer winters there are, or more high scores there are on a certain pinball machine, the less likely it is to be due to the chaos inherent in the system.

To account for the influence of chaos, climate scientists run the models repeatedly, with slightly different starting conditions. The difference in outcomes gives scientists an indication of the uncertainty in any given prediction, of the range of possible outcomes.

The validity of models can be tested against climate history. If they can predict the past (which the best models are pretty good at) they are probably on the right track for predicting the future – and indeed have successfully done so.

Clouded judgement
Climate modellers may occasionally be seduced by the beauty of their constructions and put too much faith in them. Where the critics of the models are both wrong and illogical, however, is in assuming that the models must be biased towards alarmism – that is, greater climate change. It is just as likely that these models err on the side of caution.

Most modellers accept that despite constant improvements over more than half a century, there are problems. They acknowledge, for instance, that one of the largest uncertainties in their models is how clouds will respond to climate change. Their predictions, which they prefer to call scenarios, usually come with generous error bars. In an effort to be more rigorous, the most recent report of the IPCC has quantified degrees of doubt, defining terms like “likely” and “very likely” in terms of percentage probability.

Given the complexity of our climate system, most scientists agree that models are the best way of making sense of that complexity. For all their failings, models are the best guide to the future that we have.

Finally, the claim is sometimes made that if computer models were any good, people would be using them to predict the stock market. Well, they are!

A lot of trading in the financial markets is already carried out by computers. Many base their decisions on fairly simple algorithms designed to exploit tiny profit margins, but others rely on more sophisticated long-term models.

Major financial institutions are investing huge amounts in automated trading systems, the proportion of trading carried out by computers is growing rapidly and some individuals have made a fortune from them. The smart money is being bet on computer models.

One of the sources of this idea may have been a 1971 paper by Stephen Schneider, then a climate researcher at NASA's Goddard Space Flight Center in Maryland, US. Schneider's paper suggested that the cooling effect of dirty air could outweigh the warming effect of carbon dioxide, potentially leading to an ice age if aerosol pollution quadrupled.

This scenario was seen as plausible by many other scientists, as at the time the planet had been cooling. Furthermore, it had also become clear that the interglacial period we are in was lasting an unusually long time.

However, Schneider soon realised he had overestimated the cooling effect of aerosol pollution and underestimated the effect of CO2, meaning warming was more likely than cooling in the long run. In his review of a 1977 book called The Weather Conspiracy: The Coming of the New Ice Age, Schneider stated: "We just don't know...at this stage whether we are in for warming or cooling – or when." A 1975 report by the US National Academy of Sciences merely called for more research.

The calls for action to prevent further human-induced global warming, by contrast, are based on an enormous body of research by thousands of scientists over more than a century that has been subjected to intense – and sometimes ferocious – scrutiny. According to the latest IPCC report, it is more than 90% certain that the world is already warming as a result of human activity.

It is certainly true that Earth has experienced some extremes that were warmer than today, as well as much colder periods. In some cases the main factors that caused these past warm periods – and the ebb and flow of ice ages over recent millennia – are well understood, though not in all. Many of the details remain unknown.

Within the past billion years, there may have been one or more periods when the whole planet was covered in ice. This "snowball Earth" phenomenon remains controversial, with some evidence suggesting that there were at least some areas of unfrozen land and water even at the height of the freezing. It is clear, though, that from about 750 million to 580 million years ago, the Earth was in the grip of an ice age more extreme than any since.

Why did it happen? The spread of ice produces further cooling by reflecting more of the Sun's energy back into space. But ice on land blocks the chemical weathering of rocks that removes CO2 from the atmosphere, which leads to warming as levels rise.

Snowball Earth may have been possible only because the continents were clustered on the equator, meaning CO2 removal would have continued even as ice sheets spread from the poles. Only when most of the land was covered would greenhouse gases have started to build up to levels is high enough to overcome the cooling effects of the extensive ice cover.

The warming, which lasted 200,000 years, was caused by the release of massive amounts of methane or CO2. It was thought to have come from the thawing of methane clathrates in deep ocean sediments, but the latest theory is that it was caused by a massive volcanic eruption that heated up coal deposits. In other words, the PETM is an example of catastrophic global warming triggered by the build-up of greenhouse gases in the atmosphere.

Since then, the Earth has cooled. For the past million years or so, the climate has switched between ice ages and warmer interglacial periods with temperatures similar to those of the past few millennia. These periodic changes seem to be triggered by oscillations in the planet's orbit and inclination that alter the amount of solar radiation reaching Earth.

However, it is clear that the orbital changes alone would not have produced large temperature changes and that there must have been some kind of feedback effect.

After the last glaciation ended, global temperatures appear to have peaked around 6000 years ago, called the Holocene Climatic Optimum. The warming appears have been largely localised, concentrated in the northern hemisphere in summer, and average global temperatures did not exceed those of recent decades by much, if at all. Again, orbital variations were the trigger, but these led to changes in vegetation and sea-ice cover that produced marked regional climatic alterations.

From about AD 800 to AD 1300, there was a minor peak called the medieval warm period, but it was not as warm as recent decades.

The details are seldom as simple as they seem at first: sea ice reflects more of the Sun's energy than open water but can trap heat in the water beneath, for example. There are complex interactions between many of these factors that can amplify or dampen changes in temperature.

The important question is what is causing the current, rapid warming? We cannot dismiss it as natural variation just because the planet has been warmer at various times in the past. Many studies suggest it can only be explained by taking into account human activity.

Nor does the fact that it has been warmer in the past mean that future warming is nothing to worry about. The sea level has been tens of metres higher during past warm periods, enough to submerge most major cities around the world.

Global warming is already happening. Just about every part of the planet, except for Antarctica has warmed since 1970. Glaciers are melting, spring is coming earlier and the ranges of many plants and animals are shifting polewards.

This does not sound too bad, and for many people it won't be. Wealthy individuals and countries will be able to adapt to most short-term changes, whether it means buying an air conditioner or switching to crops better suited to the changing climate. Rainfall will fall in mid-latitudes but rise in high latitudes, and initially agricultural yields will probably increase (see Higher CO2 levels will boost plant growth and food production). Some regions will suffer, though. Africa could be hardest hit, with yields predicted to halve in some countries as early as 2020.

Frequent bleaching

Wildlife will also be in trouble. Some plants and animals will thrive as CO2 rises but at the expense of others. Coral reefs, which are already suffering frequent bleaching episodes, could be particularly hard hit. Many species, like the polar bear, will suffer as their habitat disappears.

As global temperature climbs to 3°C above present levels - which is likely to happen before the end of this century if greenhouse emissions continue unabated - the consequences will become increasingly severe. More than a third of species face extinction. Agricultural yields will start to fall in many parts of the world. Millions of people will be at risk from coastal flooding. Heatwaves, droughts, floods and wildfires will take an ever greater toll.

There are two factors should borne in mind when thinking about the impacts. Firstly, even countries that escape the worst of the direct effects will feel the economic effects of what happens elsewhere. There may be social and political problems too, as migration increases and water becomes increasingly scarce in some regions.

Time lags

Secondly, there are time lags between rises in CO2 and their impact on climate. These lags mean that the longer we delay effective action, the more severe the impacts will eventually be.

There is a lag between CO2 rises and their full effect on global temperature. Even if we made the drastic cuts necessary to stabilise CO2levels tomorrow, the world would continue to warm for decades.

There is an even longer lag between any increase in temperature and the resulting rise in sea level. The IPCC is predicting a rise of 0.6 metres at most by 2100 but this will just be the start.

The IPCC predicts a minimum temperature rise by 2100 of 1.8°C. About 120,000 years ago, when it was 1 to 2°C warmer, the sea level was 5 to 8 metres higher - more than enough to inundate many major cities around the world, including New York, London and Sydney. Three million years ago, when the temperature was 2 to 3°C higher, it was 25 metres higher.

There is no doubt that similar temperature increases will eventually lead to similar rises in sea level. The assumption is that it will take many centuries, as the Greenland and Antarctica ice caps slowly melt and the oceans expand as the waters warm. But some researchers think it could happen much sooner due to the sudden collapse of ice sheets.