- 16 May 2007
- NewScientist.com news service
- Catherine Brahic
- David L. Chandler
- Michael Le Page
- Phil McKenna
- Fred Pearce
Myth: Carbon dioxide levels only rose after the start of warm periods, so CO2 does not cause warming
Samples of ice dating back hundreds of thousands of years have been extracted from the sheets covering Antarctica and Greenland. These cores show that at the end of recent ice ages, the level of CO2 in the atmosphere often did not start to rise until temperatures had already been climbing for some time. There is uncertainty about the precise timing, partly because the air trapped in the cores is younger than the ice itself, but it appears the lags might sometimes have been 800 years or more.
These lags show that rising CO2 did not trigger the initial warming at the end of these ice ages - but then, no one claims it did. They do not undermine the idea that more CO2 in the atmosphere warms the planet.
We know CO2 is a greenhouse gas because it absorbs and emits infrared. Fairly basic physics proves that such gases will trap heat radiating from Earth, and that the planet would be a lot colder if this did not happen.
This does not mean that there will be a perfect correlation between past temperature and past CO2 levels. Many other factors also affect the climate: when there are big changes in these factors, the relationship between CO2 and temperature will be obscured.
So why, over the past million years or so, has Earth repeatedly switched between ice ages and warmer periods? The long-held theory is that this is due to variations in Earth's orbit - known as Milankovitch cycles - that change the amount and location of solar energy reaching Earth. These correspond with most - but not all - climate transitions (see Graph). However, their direct heating or cooling effect is small, and does not fully explain the temperature switches.
This suggests that some kind of feedback effect amplified the initial changes in temperatures. The ice itself is one contender here. As vast ice sheets started to shrink, less of the sun's energy would have been reflected back into space, accelerating the warming.
The possibility that CO2 also plays a role was suggested more than a century ago. The ice cores show that there is a remarkable correlation between CO2 levels and temperature over the past half-million years. It takes about 5000 years for an ice age to end and, after the initial lag, temperature and CO2 concentrations in the atmosphere rise together for at least 4000-odd years.
What seems to have happened at the end of ice ages is that an initial warming due to orbital shifts led to more CO2 being released into the atmosphere, resulting in further warming that caused still more CO2 to be released and so on. As the area of ice shrank, temperatures rose still higher.
Where did the extra CO2 come from? The evidence suggests it was from the oceans. The gas is less soluble in warmer water, so warmer seas release it into the air, but this can explain only a little of the increase. Another factor may have been biological: phytoplankton in the seas soak up CO2 as they grow and fall to the ocean floor, but as the world warmed changes in winds, currents and salinity would have cut the phytoplankton's growth.
While CO2 was only a secondary player in the ice ages, further back in time there are examples of warming triggered by rises in CO2 (see below). What the ice ages tell us is that temperature can influence CO2 levels as well as vice versa, which is a cause for concern. At the moment, the oceans are soaking up 40 per cent of the extra CO2 we are emitting. If they switch to emitting CO2 instead, cuts in human emissions will make little difference.
Half-truth: It has been warmer in the past, so what's the big deal?
FIRST, it needs to be said that everything we think we know about global temperatures before about 150 years ago is an estimate - a reconstruction based on second-hand evidence such as ice cores and a set of assumptions. The further back we look, the greater the uncertainties.
It is certainly true that Earth has experienced some extremes that were warmer than today. In some cases the main factors that caused these climatic variations are well understood, though not in all.
From 750 million to 580 million years ago, Earth was in the grip of an ice age more extreme than any since. At times the whole planet may have been covered in ice and snow, a phenomenon known as Snowball Earth.
Why did this happen? The balance between two opposing effects may have been crucial. The growth of ice sheets can lead to extra cooling as more of the sun's heat gets reflected back into space. However, ice on land blocks the weathering of rocks, a process that removes CO2 from the atmosphere. Snowball Earth may have come about because the continents were then clustered on the equator: weathering would have continued to remove CO2 even as ice sheets spread from the poles. Only when most of the land was iced over would greenhouse gases have started to build up.
After this deep freeze, there were long periods when both greenhouse gas levels and temperatures were higher than they are today, though there is great uncertainty about the details (see Graph). The warmest period was probably the Palaeocene-Eocene Thermal Maximum (PETM) about 55 million years ago. During this event, which coincided with mass extinctions, global temperatures may have warmed by 5 to 8 °C within a few thousand years. The Arctic Ocean reached 23 °C.
Isotope levels in fossil plankton show the warming was caused by the release of massive amounts of methane or CO2. The latest theory is that this was due to lava from a massive volcanic eruption heating coal deposits. In other words, this may be an example of catastrophic global warming caused by the sudden release of massive quantities of fossil carbon into the atmosphere. The warm period lasted 200,000 years.
Over the past few million years Earth has switched between ice ages and warmer interglacials. These periodic changes seem to be triggered by oscillations in the planet's orbit that alter the amount of solar radiation reaching Earth (see Graph).
In between ice ages, there have been several temperature peaks, notably during the Eemian interglacial around 125,000 years ago. At this time, temperatures may have been 1 to 2 °C warmer than today, and the sea level was 5 to 8 metres higher than it is now.
"During the Eemian, it was 1 °C warmer and sea level was 5 to 8 metres higher
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After the last ice age, there was another peak around 6000 years ago called the Holocene Climatic Optimum. This warming appears to have been largely regional, though, and temperatures were probably not much higher than in recent decades, if at all.
Do these past periods of natural warming mean we can dismiss the rapid warming over the past few years as more of the same? The answer is no. Natural factors such as changes in the amount of solar energy reaching the Earth can explain only a small part of the recent warming.
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 many major cities.
Half-truth: Human carbon dioxide emissions are tiny compared with natural sources
YES, it's true that CO2 emissions due to human activity are small compared with most natural sources. Yet ice cores show that levels in the atmosphere have remained fairly steady at between 180 and 300 parts per million for the past half-million years, only to shoot up to more than 380 ppm since the industrial age began.
How is this possible? The answer is that natural sources are balanced by natural sinks (see above). The breakdown of organic matter, for instance, releases huge quantities of CO2, but growing plants soak up just as much. CO2 levels have risen because slightly more of the gas has been entering the atmosphere each year than can be soaked up by natural sinks.
How can we be sure that we are responsible for the extra CO2? There are several lines of evidence. For instance, fossil fuels contain virtually no carbon-14, because this unstable isotope, formed when cosmic rays hit the atmosphere, has a half-life of around 6000 years. Nearly all the carbon-14 in a fossil fuel will have long decayed by the time we burn the fuel, so the resulting CO2 will contain almost no carbon-14 too. Studies of tree rings have shown that the proportion of carbon-14 in the air dropped by about 2 per cent between 1850 and 1954 (after 1954, nuclear tests released large amounts of carbon-14).
Finally, claims that volcanoes emit more CO2 than human activities are simply not true. CO2 levels around the world do not rise after major eruptions. Total emissions from volcanoes on land are estimated to average just 0.3 gigatonnes of CO2 each year - about a hundredth of human emissions - and are balanced by the carbon carried under tectonic plates in subducted ocean sediments.
"Claims that volcanoes emit more CO2 than human activities are not true
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Myth: It's too cold where I live. A bit of warming will be great
HOW will climate warming affect you? It depends on where you live, how long you live, what you do for a living and for recreation - and whether you care about the future of your children, or humanity in general.
Just about every part of the planet except Antarctica has warmed since the 1970s. Glaciers are melting, spring is coming earlier and the ranges of many plants and animals are shifting polewards.
For most people, this has made little difference. We may have sweltered through more heatwaves, but winters have been milder. The next decade or two will also be a mixed bag. Heating bills will go down, air conditioning bills will go up. Heatwaves may cause some deaths but there will be fewer cold-related deaths.
This does not sound too bad, and for many people it won't be. In cooler regions the benefits could outweigh the downsides, depending on your point of view. 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 a warmer climate and changing rainfall patterns. Overall, agricultural yields could increase at first. Some regions will suffer, however, and soon: Africa will fare worst, with yields predicted to halve in some countries as early as 2020.
Wildlife will also be in trouble. Certain plants and animals will thrive as CO2 rises, but at the expense of others. Coral reefs, which are already suffering frequent bleaching episodes, will be especially hard hit.
Things will become increasingly dire as temperatures climb to 3 °C above present levels, which could happen long before the end of the century in the worst-case scenario. More than a third of species will face extinction. Agricultural yields will fall in most parts of the world. Millions will be at risk from coastal flooding. Heatwaves, droughts, floods and wildfires will take an ever heavier toll.
There are two factors to bear in mind when thinking about the outcomes of warming. Firstly, even countries that escape the worst direct effects will feel the economic and political fallout from what happens elsewhere. Secondly, there is a time lag between a rise in greenhouse gases and their full effect on climate. Even if CO2 levels were stabilised tomorrow, the world would continue to warm for decades.
There is an even longer lag between any warming and its full effect on sea level. The IPCC is predicting a rise of 0.6 metres at most by 2100, but this will be just the start. Three million years ago, when the temperature was 2 to 3 °C higher, sea level was 25 metres higher - more than enough to inundate New York, London, Tokyo and Shanghai. A similar temperature increase will eventually lead to a similar rise in sea level. The IPCC assumes this will take many centuries, but some think it could happen much sooner due to the catastrophic collapse of ice sheets.
What's clear is that the longer we delay effective action, the harder it will be to prevent catastrophic climate change.
Myth: It's all down to cosmic rays
NO ONE denies the crucial influence of the sun on Earth's climate. The total amount of energy reaching Earth varies, but recent variations cannot explain the recent warming. What if changes in other forms of solar activity have larger-than-expected effects on the climate, though?
In the late 1990s, Danish scientists revived the idea that the high-energy particles known as cosmic rays might influence cloud formation by ionising the atmosphere. If so, this could amplify the effect of small changes in solar activity on the climate. Though most cosmic rays come from deep space, changes in solar activity can alter the number that reach Earth. When there are many sunspots, the sun's magnetic field strengthens, deflecting more of the cosmic rays in the solar system.
Henrik Svensmark of the Danish National Space Center claims that fewer cosmic rays would mean fewer clouds, so warming Earth. He thinks this effect explains the recent warming, arguing the case in a book he wrote with science journalist Nigel Calder (who edited New Scientist from 1962 to 1966).
There are at least three separate issues here. Firstly, do cosmic rays really trigger cloud formation? Secondly, if they do, how do the changes in cloud cover affect temperature? Finally, can this explain the warming trend of the past few decades?
The hypothesis is that the ionisation of air by cosmic rays imparts an electric charge to aerosols that encourages them to clump together; the clumps become large enough to trigger the condensation of water, and hence clouds form. As yet there is no convincing evidence that such clumping occurs. Experiments under way at the CERN particle physics laboratory near Geneva should settle the issue, but will not reveal if it matters in the real world: the atmosphere already has plenty of cloud condensation nuclei, so it is not clear why cosmic rays should have any great effect on cloud formation.
A series of attempts by Svensmark to show an effect have come unstuck. Most recently, he has claimed there is a correlation between low-altitude cloud cover and cosmic rays. Yet a correlation does not prove cause and effect. What's more, the correlation holds up after 1995 only if data is "corrected", and others in the field say this correction is not justified (see "A cosmic connection?"). "It's dubious manipulation of data in order to suit his hypothesis," says Joanna Haigh, an atmospheric physicist at Imperial College London, UK. A few independent studies by other groups hint at a very tiny effect on clouds, but most have found no effect.
Then there is the question of how clouds and climate interact. Svensmark claims the overall effect of less cloud cover is a warmer world in which the extra heat that clear skies allow in during the day outweighs the increased heat losses at night. Not all scientists agree with this reasoning, as even during the day many clouds in the upper atmosphere can in fact have a warming effect.
Finally, and most importantly, even if changes in cosmic ray intensity do turn out to influence cloud cover and temperature, they cannot explain the rapid warming of the past few decades. Direct measurements going back 50 years show a periodic variation in intensity, but no downward trend coinciding with the recent warming (see main graph).
Indirect measurements of cosmic rays, based on the abundance of certain isotopes, suggest that their intensity fell between 1900 and 1950. While there can be a lag between a big change in a climate "forcing" and its full effect on temperature, most warming should occur within a few years and taper off within decades. This is not the pattern we see.
Half-truth: Antarctica is getting cooler and the ice sheets are getting thicker
IT IS clear that the Antarctic Peninsula, which juts out from the mainland, has warmed. The continent's interior was thought to have warmed too, but in 2002 an analysis of records from 1966 to 2000 concluded that it had cooled.
This is not, as sometimes claimed, proof that the world is not warming. Climate models do not predict uniform warming of the whole planet, and almost every other part of the world is getting warmer.
The cooling in Antarctica is due to a strengthening of the circular winds around the continent, which prevents warmer air reaching the interior. Confusingly, the increased wind speeds seem to be due to cooling in the upper atmosphere caused by the hole in the ozone layer above the pole - the result of chlorofluorocarbon emissions. If the ozone layer recovers over the next few decades as expected, the circular winds could weaken, resulting in rapid warming.
This raises the question of what is happening to Antarctica's ice sheets, which hold enough water to raise sea levels by a catastrophic 61 metres. Contrary to what you might expect, the latest IPCC report continues to predict that global warming will lead to a thickening of the ice sheet over the next century, with heavier snowfall outweighing any melting.
Finding out what is actually happening to the ice is not easy. A recent study based on satellite measurements of gravity over the continent suggests that while the ice sheets in the interior of Antarctica are growing thicker, even more ice is being lost from the peripheries, resulting in a net loss.
The IPCC's latest predictions of sea level rise - 20 to 60 centimetres by 2100 - assume that the rate of ice loss from the edges of both the Greenland and Antarctic ice sheets continues at the current rate. Some researchers think this is unrealistic and that the ice loss will accelerate, outpacing any increases in snowfall and leading to a much more rapid rise in sea level. No one knows for sure what will happen.
Myth: It was warmer during the Middle Ages than it is now, with vineyards in England
ENGLISH winemaking is once again thriving: the extent of the country's vineyards probably surpasses that in the so-called Medieval Warm Period. So if you think this is an accurate indicator of climate, it must be warmer now than it was then.
Historical anecdotes about climate have to be treated with caution. The frost fairs that were held in London when the Thames froze over are sometimes hailed as proof of how cold it was during the Little Ice Age from around AD 1500 to 1850. In fact, the slowing of the river by the old London Bridge, demolished in 1831, was a crucial factor in its freezing - which is why the Thames did not freeze in London in the winter of 1963, even though it was the third-coldest in England since 1659.
To work out how average global temperature has changed over the centuries, climate scientists need long-term records from as many different parts of the world as possible, which is why they have turned to indicators such as growth rings in trees. There are now a dozen or so temperature reconstructions for the northern hemisphere that go back beyond 1600. These studies show periods of unusual warmth from around AD 900 to 1300, but the details vary.
In the southern hemisphere, there is evidence of both warm and cool periods around this time. This suggests the Medieval Warm Period was partly a regional phenomenon, caused by a redistribution of heat around the planet as well as a small rise in the average global temperature.
The reconstructions and other evidence show that the planet is warmer now than at any time during the medieval period (see left). What really matters, though, is not how warm it is now, but how warm it's going to get in the future. Even the reconstructions that show the greatest variations suggest that average temperatures remained within a narrow band right up to the 1980s. Now we are out of that band and climbing fast.
From issue 2604 of New Scientist magazine, 16 May 2007, page 34-42
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