Stripping Down Science (4 page)

The volunteers were presented with plates of
biscuits to eat, either one hour or three hours after they were given a set lunch at the laboratory. Before embarking on the biscuit sampling, half of the students were asked to produce a detailed description of the lunch they had eaten, while the other half were asked to write down the details of their journey in to the test that day. After some cookie-related questions intended to mask the true nature of the trial, the participants were invited to eat as many of the remaining biscuits as they wanted. The team then weighed the plates to find out how much each of their subjects had consumed.

Although there were no differences in how hungry they claimed to be, the students who recalled lunch, Higgs found, ate about a third fewer biscuits than those asked to recollect their journey to the lab. The effect was also much more pronounced when the students were tested three hours after lunch compared to when the test was carried out one hour post-lunch.

The researchers think that this is probably because the memory of eating lunch was still vivid enough at the one-hour stage to affect the women's appetites, regardless of what they were asked to recall. By three hours, though, those
memories had faded, except in the participants who reminded themselves of what they'd eaten. For them, recalling their last meal did make a difference to how hungry they felt.

So why do we normally associate thinking about food with feeling hungry? Because, say the researchers, this involves thought about food in general. The difference in this study was that the subjects were thinking about a specific meal and how this relates in time to their present situation. Replaying that memory has the power to alter how the brain weighs up appetite, translating into reduced food intake, even if we don't feel less hungry.

Now the researchers want to know whether the process also works the other way around. ‘I'm following up with a study that examines the theory in reverse, so whether disrupting people's memories while eating – by watching television whilst eating, for example – causes an increase in appetite,' says Higgs. She didn't, however, mention the possible effects of watching a television show about cooking …

A big scientific mystery is how the early earth, four and a half billion years ago, managed to remain so warm and was an ideal place to spawn life, despite the young sun pumping out 30% less light and heat than it does today. Moreover, as the sun has warmed up since, why has the world remained at a steady temperature?

Scientists thought that the answer to this ‘faint young sun' paradox was that the planet was initially blanketed in a thick shroud of CO
₂
that, through a greenhouse effect, kept the planet artificially warm. But new research suggests that this CO
₂
story is a myth, if not a load of hot air, and that the real answer is much more cunning. In questioning, 40 years ago, how the world kept itself warm when it first formed, the American space scientist Carl Sagan effectively threw a planet-sized spanner into the cosmological works. He pointed out that when the solar system first assembled itself, the low level of output from the immature sun would have left earth in the deep freeze and certainly
far too cold for anything other than ice to exist.

Yet the geological record paints a very different picture. Written into ancient rocks are clear signs that the earth was bathed in large amounts of water and experienced stable temperatures of 70 degrees Celsius or so. Because there wasn't much oxygen around during this time, scientists initially suggested that a dense smog of methane and ammonia was responsible for trapping more heat from the sun and keeping the temperatures up. But when they realised that ammonia breaks down in sunlight, they gave up on that answer.

The problem was finally thought to have been solved when the American scientist James Kasting
¹³
suggested in the early 1990s that CO
₂
might be the answer. By acting as a greenhouse gas, and with an atmospheric concentration approaching 30% (70 times today's levels), this, together with water vapour, could keep the earth's temperature on track. Kasting provoked a big scientific sigh of relief and, for a while, everyone was happy with his explanation. But then geochemists unearthed ancient rocks that seemed to show that the CO
₂
levels were actually far lower than would be
required and the jury was out again.

Now University of Copenhagen scientist Minik Rosing and his colleagues
¹⁴
think they have solved the mystery once and for all. They've analysed compounds of iron found in rocks more than 3.8 billion years old. These so-called banded iron formations contain two different iron minerals, magnetite and siderite, which form in different ratios according to how much CO
₂
is around. The results clearly indicate that there couldn't have been much more than about three times the present-day levels of CO
₂
in the ancient atmosphere, which is a far cry from the 30% that would be needed if CO
₂
was the answer.

Instead, their calculations suggest that the effect is down to albedo, which is the amount of solar energy reflected off the planet's surface and back into space. On the early earth, the continents were much smaller, most of the planet's surface was occupied by heat-hungry water, and there were fewer reflective clouds in the sky owing to a lack of sulphur and other compounds that normally trigger cloud formation. Together, these effects meant that, during these early times,
far less of the sun's heat was bounced back into space, so the planet simmered beautifully.

But why didn't the world continue to warm as the sun matured and began pumping out more heat? Because, the researchers show, as this was happening, the continents grew, reflecting light back into space. Life appeared, modifying the atmosphere, and more light-reflective clouds formed in the sky. This, they say, all adds up to the balmy, stable temperatures we enjoy today.

Apart from solving a longstanding mystery, these results are also important, say the scientists, because they show that, contrary to popular belief that CO
₂
levels in the past have been much lower than they are today, atmospheric CO
₂
concentration appears to have remained relatively stable throughout the lifetime of the earth.

So, when designing computer models to predict potential future climate changes based on past measures of CO
₂
, scientists need to take this into account. Otherwise, to paraphrase the famous Spanish philosopher George Santayana, if we don't learn from history, we're doomed to repeat it.

FACT BOX

Why we can't rely on trees to combat climate change

We tend to regard trees as allies in the fight against global warming because they lock away carbon dioxide from the atmosphere and encourage the formation of clouds, which help to reflect heat back into space and keep the earth cool. This is part of the argument put forward by Minik Rosing to solve Carl Sagan's paradox (see ‘There was no greenhouse effect!').

Unfortunately, scientists have now discovered that when the temperature rises, plants up their output of a class of chemicals that cut cloud formation, potentially accelerating, rather than mitigating, global warming. Normally, microscopic airborne particles called CCNs – cloud condensation nuclei – encourage airborne water vapour to condense into droplets by providing a surface upon which they can form. A major source of these CCNs are volatile chemicals called monoterpenes, which are released in large
amounts by coniferous trees.

Airborne monoterpenes react with ozone and also with hydroxyl (OH) radicals, both of which are naturally found in the air at low levels, to produce the chemicals that ultimately link up to form CCNs. Predictably, the greater the concentration of CCNs, the greater the number of water droplets that can form and the smaller they tend to be, which means more clouds to reflect heat and light back into space, helping to offset global warming. And since plants produce more monoterpenes in the summer time, scientists had assumed that, if global temperatures rise, trees would stay in step and increase their output too. This, in turn, would lead to more clouds and more cooling, helping to reset the balance.

But now Astrid Kiendler-Scharr, a researcher based at the Jülich Research Centre in Germany,
¹⁵
has found a serious fly in the atmospheric ointment. Using a special sealed chamber to monitor the chemicals
being produced and consumed by plants as they grow, she has found that deciduous trees also pump out a small hydrocarbon molecule called isoprene, chemical formula C
₅
H
₈
. And the warmer it is, the more isoprene the trees make. The problem is that this also reacts with the hydroxyl (OH) radicals in the air, so it competes with the monoterpenes that are trying to form CCNs, meaning fewer clouds and therefore less heat reflection back into space, which could accelerate global warming.

Scientists are also worried because, as well as boosting isoprene levels, another study has found that higher temperatures could also transform the Amazon rainforest from a prodigious carbon consumer into a massive carbon dioxide source. Leeds University ecologist Professor Oliver Phillips, together with an international team of more than 60 collaborators,
¹⁶
looked at 136 plots of rainforest over a number of years to work out how much biomass (biological matter) was present and
therefore how much carbon the Amazon was actually locking away every year. They found that, in the years leading up to 2005, the Amazon was a powerful carbon ‘sink', tying up more than a tonne of carbon per hectare of rainforest per year.

But in 2005 there was a severe drought that led to the death of many trees and growth arrest among the survivors. This meant that, as the dead material began to break down, and without fresh carbon being locked away by growth, patches of the forest began losing up to two tonnes of carbon per hectare per year. The significance of this result is that the 2005 drought was provoked by warmer-than-normal water in the north Atlantic, which is what triggered Hurricane Katrina and led to the flooding of parts of New Orleans. Unfortunately, it had the reverse effect over the Amazon, and if global warming continues we might therefore see a drier Amazon more often.

This would mean that the billions of tonnes of carbon locked away by the rainforest every year would cease to be removed from
the atmosphere, while at the same time the Amazon would become a net carbon source as the existing biomass broke down. The result would be a dramatic acceleration of the greenhouse effect, with predictable global consequences.

This should provoke a re-think of the political agenda,' says Phillips.

Chameleons have earned a reputation for being masters of disguise because of their incredible ability to change body colour within seconds. But it's a myth that they do so in order to blend in with their surroundings. In fact, the main reason chameleons change colour is so that they can communicate with each other and regulate body temperature. A calm chameleon, for example, is usually pale green, while an angry one turns bright yellow. A cold creature often takes on a darker hue to soak up more sun, while a speed dating chameleon, with mating on its mind, usually sports an explosive burst of reds, greens, browns, whites and blues. But how do they perform the lizard equivalent of a total body makeover in just a few seconds?