IT HAS been another “normal� global-warming summer in the northern hemisphere. The US sweltered in the , following the . More than 60 per cent of the contiguous US is suffering from drought, as are parts of eastern Europe and India. In the Arctic, sea ice cover is at a record low and the Greenland ice sheet shows what the calls “extraordinary high melting�. Global land temperatures for and were the hottest since records began in the 19th century.

Meanwhile, El Niño conditions are , warming up ocean surface temperatures. Some observers have predicted that this will lead to record-breaking global temperatures next year.

If El Niño does arrive and temperature records are broken, there will inevitably be much discussion of the causes of the warming. So now is a good time to sort signal from noise in the global temperature records.

For the past 30 years, global temperature has shown a linear warming trend of 0.16 °C per decade (). When looking for the cause of this warming, a physicist will look for the heat source. One possibility is that the oceans are releasing heat. But measurements show the opposite: the oceans are soaking up heat. The other possibility is that the heat is coming from above, and indeed it is: more radiation is entering the top of the atmosphere than leaving it. This is because increasing amounts of greenhouse gases in the atmosphere hamper the loss of heat into space.

Superimposed on this global-warming signal is short-term natural variability, which makes some years hotter and some colder. Some, notably 2005 and 2010, stick out above the trend line, whereas others, like 2008 and 2011, stay below it. But overall, temperatures are creeping upwards within a corridor of plus or minus 0.2 °C around the trend line. Climate deniers use this variability to claim there is a slowdown in global warming, by cherry-picking time intervals that happen to start in the upper part of the corridor and end in the lower. They mix up signal and noise.

Three known factors explain much of the natural variation. The first is volcanic eruptions – the eruption of Mount Pinatubo in the Philippines in 1991 was followed by three cold years, for example. Then there is the sun’s variability, mostly in the form of the 11-year sunspot cycle. Finally, there is the irregular oscillation between warm El Niño and cold La Niña conditions in the Pacific.

We have independent measurements describing all three that we can easily correlate to global temperature changes. This shows, for example, that during a solar maximum the globe is about 0.1 °C warmer than during a solar minimum, but also that solar activity has contributed nothing to the warming trend of the past 30 years. In fact, it has acted to reduce it, but the effect is so small that the hottest year on record, 2010, was near the end of the deepest solar minimum since satellite measurements began in the 1970s.

The analysis further shows that global temperature typically reaches a maximum about four months after El Niño conditions peak, and is correspondingly colder after La Niña. La Niña episodes in 2008 and 2011 have cooled the past few years, masking the warming trend. But while 2011 was cool in the context of the previous 10 years, it was the hottest La Niña year on record.

It is straightforward to remove the effects of the solar and El Niño cycles from the data, just as unemployment figures routinely have seasonal effects removed. Once this is done, and regardless of the global temperature dataset used, the result is always a steady warming trend that has been no slower in the past decade than it was in the previous two – and which, incidentally, agrees with what is predicted by the Intergovernmental Panel on Climate Change.

Now solar activity is on the way back up and it is only a matter of time before the next El Niño event comes along. In fact, predictions by the US National Oceanic and Atmospheric Administration suggest that El Niño conditions are likely to arrive any time now. These two factors, combined with the ongoing warming trend, make it likely that a global temperature record will be set next year – unless a major volcano erupts.

Perhaps a record year will silence those unscientific voices who claim that global warming has come to an end. But the denial industry has already come up with a plan B: to claim that global warming is completely down to El Niño. To expose the fallacy of that, we just need to look again at where the heat comes from: below or above.

In the case of El Niño, the warmth comes from the ocean. During El Niño events, the global ocean releases heat, whereas during La Niña events, it recharges its heat store. That is confirmed by satellite measurements of the radiation balance: during recent La Niña events our planet did not lose heat to space. On the contrary, it absorbed more than normal. That is to be expected: when the ocean exposes colder waters at its surface, as during La Niña, these soak up extra heat.

So if global warming of the past decades was due to El Niño or another mechanism involving heat from the ocean, the ocean would have lost heat. But the heat content has gone up, not down. And it is well understood why: because we created a radiation imbalance by adding greenhouse gases to our atmosphere.

The signal of global warming caused by humans is very clear, despite attempts by certain parties to drown it out with a lot of noise.

Some people seem to think that science should provide the facts and
that the debate about the consequences should be left to the political arena.
But life is not so simple. ÐÓ°ÉÔ­´´s need to explain the significance of
their results to the general public and the decision makers, and in this
case the significance is highly political. It is not enough just to publish
technical papers, hoping that the politicians will figure out for themselves
what it all means. If a few scientists had not jumped up and down urging
for action, nothing would have been done to face the greenhouse threat.
Yet within the scientific community this sort of behaviour is still frowned
upon.

One problem with greenhouse science is that there is little hard fact.
Most statements are statistical in nature. For example, we cannot say that
a particular storm has been caused by the greenhouse effect. We can only
say that the average number and the intensity of storms are expected to
increase in a warmer world. Even the calculation of a seemingly simple parameter
like the global average temperature is a statistical problem fraught with
difficulties. Forecasts of what the climate may be like in the future rely
by necessity on computer models, since we cannot perform experiments on
the real Earth. We can say that the computer models represent our state-of-the
art knowledge, but we cannot be sure that we have not missed some effect.

Despite all this, the combined evidence from models, studies of climates
long past and recent observations makes a strong case that significant global
warming will result from the way our industrial society is changing the
composition of the atmosphere. The poor scientific quality of the arguments
put forward by the ‘dissenters’, who dismiss any risk of global warming,
confirms that the evidence for global warming must indeed be strong. And
a debate on whether the likelihood of a severe climate disaster is 50
per cent or 80 per cent is actually quite pointless. Imagine planning a
holiday and being told that, unfortunately, your plane has a high chance
of crashing. Would you bother to find out whether the odds are 50 per cent
or 80 per cent, or would you rather go by train, even if it were to mean
a slower and less comfortable journey?

It is sometimes forgotten that the uncertainties in the global warming
forecasts can go both ways. The impacts could also be more severe than the
models predict, and we might be in for more unpleasant surprises like the
opening of the Antarctic ozone hole, which no one predicted.

The best way to deal with uncertainty and differing judgements is an
open debate, and the Intergovernmental Panel on Climate Change (IPCC) has
provided a truly international forum for this, helping to clarify what we
know and what we do not know. Its most recent report confirms the earlier
estimates that our planet could warm by between 2 and 4 degree C over the
next century. What is remarkable about the IPCC is the consensus that was
reached among scientists from many countries, not the fact that there are
a few who disagree. Even more remarkable, perhaps, is the fact that politicians
are beginning to take note. Probably the largest collection of heads of
state in history gathered in Rio last year to sign the climate convention.
I suspect that only 10 years ago few scientists would have imagined their
graphs and computer printouts could have such political impact.

Now that measures to combat global warming are seriously being considered
by many governments, a different category of computer model has entered
the arena: the economic model. These are presently used mainly by those
who oppose any reductions in the emission of greenhouse gases. They can
help to shroud a political statement in complex terms and thus give it a
certain ring of credibility. If one reads the small print, however, it is
noticeable that most of these models assume that the cost of a 3 degreeC
warming would only be 1 or 2 per cent of today’s gross world production.
Not surprisingly, these models always seem to show that it is best to do
nothing at all about global warming, and to keep increasing our pollution
of the atmosphere. (In the economist’s jargon, this is called the ‘optimum
emissions pathway’.)

This has to do with the peculiar way in which economists measure wealth:
environmental degradation can actually boost economic growth. Each time
there is an oil tanker accident, GNP goes up. Some climate modellers have
become dissatisfied with this and are working on a more comprehensive approach.
A group at the Max Planck Institute for Meteorology in Hamburg has already
taken the first steps towards producing a model of the ‘Global Environment
and Man’. This would include submodels of the physical climate system, natural
ecosystems, agricultural and economic systems. The aim of such a model would
be to find the best political strategy to maximise a ‘global welfare function’.
The crux lies in how global welfare will be defined and by whom.

Conventional economic models avoid facing up to the complexities of
this issue by just considering material values of marketable products. In
their calculations, what is the value of the plant and animal species threatened
with extinction through a warming of the Earth? And what is the value of
the life of a poor farmer’s family in Bangladesh, which is not acting as
consumer on the world market and whose existence might be destroyed by increased
cyclones? And how would this ‘cost’ compare with the ‘cost’ of a European
family not buying a second car, thus hampering economic growth? (They might
take to cycling instead, and enjoy it, with negative consequences for growth
in the health industry.) The global welfare function at least tries to
overcome this short-sighted attitude, but the problems of defining it rigorously
enough to be of any use for modelling will be formidable.

Another colourful facet of the greenhouse debate where science is important
is the geoengineering issue. Should we deliberately intervene in the climate
system to counteract global warming? Suggestions include fertilising the
oceans to increase their uptake of carbon dioxide through the biological
pump, shielding the Earth from the Sun with mirrors of aluminium foil orbiting
in space, and increasing the reflectiveness of the Earth (its so-called
‘albedo’) by dumping large amounts of sulphur dioxide in the stratosphere.
Proponents argue that these options would be cheaper than reducing our greenhouse
gas emissions. But some also acknowledge the vexing, legal and ethical questions
that such actions would raise. Who would be liable should unforeseen side
effects occur? Injecting SO₂ into the stratosphere could deepen
the ozone hole. Solar shields in space could be used to manipulate the weather
or even as a weapon. Fertilising the oceans could lead to toxic algal blooms.

It is unlikely that the world would ever reach a consensus to go ahead
with any geoengineering scheme, other than ‘natural remedies’ such as the
planting of forests. But there is a danger that politicians might seize
on the prospect of a cheap technical fix as an excuse to avoid tackling
the root cause of the greenhouse problem – our overconsumption and waste
of energy.

Jim Lovelock, father of the ‘Gaia’ theory, has coined the term ‘geophysiology’
for a science that is concerned with the health of the Earth – a kind of
planetary medicine. If we consider our planet as Gaia, a complex living
organism, geophysiology surely is a more fitting concept than geoengineering.

We might learn something from human medicine. If a patient is made sick
by an unhealthy lifestyle then prescribing drugs or performing an operation
may give temporary relief, but only a change from the damaging behaviour
offers a way of curing the problem in the long-term. We cannot afford to
try risky operations with unknown side effects. We only have one Earth.

Stefan Rahmstorf is based at the Institute of Oceanogtaphy at the University
of Kiel.