TWENTY
THE THREE TIPPING POINTS

Scientists are aware of three big tipping points for Earth’s climate: a slowing or collapse of the Gulf Stream; the death of the Amazon rainforests; and the explosive release of methane from the sea floor.

All three occur in the virtual worlds of the computer models, and there is some geological evidence for all having happened at various times in Earth history. Given the current rate and direction of change, one, two or perhaps all three may take place this century. So what leads to these sudden shifts, what are the warning signs, and how might they affect us?

SCENARIO 1
Collapse of the Gulf Stream

The importance of the Gulf Stream to the Atlantic rim countries is enormous. In 2003 the Pentagon commissioned a report outlining the implications for US national security should the Gulf Stream collapse. The purpose of the report was, its authors said, ‘to imagine the unthinkable’.

In their scenario the Gulf Stream slows as a result of fresh water from melting ice pouring into the North Atlantic. The planet continues to warm until 2010, but then a dramatic shift will occur—a ‘magic gate’ that will abruptly alter the world’s climate.

The Pentagon ‘weather report’ for 2010 predicts persistent drought over critical agricultural regions, and a plunge in average temperatures of more than 3°C for Europe, just under 3°C for North America, and 2°C increases for Australia, South America and southern Africa.

The report predicts that nations won’t co-operate with each other in the face of the disaster: mass starvation would be followed by mass emigration. Regions as diverse as Scandinavia, Bangladesh and the Caribbean would become incapable of supporting their populations. New political alliances would be forged in a scramble for resources. And war would be likely.

By 2010–20, with water supplies and energy reserves strained, Australia and the US would focus increasingly on border protection to keep out the migrating hordes from Asia and the Caribbean. The European Union, the report says, may go one of two ways—either it would focus on border protection (to keep out those homeless Scandinavians, among others), or be driven to collapse and chaos by internal squabbling.

In 2004 the Hollywood disaster movie The Day after Tomorrow also imagined the consequences of the shutdown of the Gulf Stream. For dramatic effect the time-lines for the collapse are greatly compressed, and the changes are far grander even than those imagined in the Pentagon report.

Scientists meanwhile have been working at understanding the consequences for biodiversity if the Gulf Stream were to collapse. They are catastrophic. If the currents will no longer carry oxygen into deeper waters, biological productivity in the North Atlantic will fall by 50 per cent, and oceanic productivity worldwide will decrease by over 20 per cent.

So what are the chances of the Gulf Stream shutting down this century? What would be the warning signs?

The Gulf Stream is the fastest ocean current in the world, and it is complex, spreading out into a series of spirals and sub-currents as its waters move northward. The volume of water in its flow is simply stupendous. You will recall that ocean currents are measured in Sverdrups, and one Sverdrup equals a flow of 1 million cubic metres of water per second. Overall, the flow rate of the Gulf Stream is around 100 Sverdrups, which is 100 times as great as that of the Amazon.

In its northern section the Gulf Stream is far warmer than the waters that surround it. Between the Faeroe Islands in Denmark and Great Britain, for example, the Gulf Stream is a balmy 8°C, yet the surrounding waters are at zero. The source of the Gulf Stream’s heat is the tropical sunlight falling on the mid-Atlantic, and the current is a highly efficient way to transport it, for one cubic metre of water can warm 3000 cubic metres of air.

In the North Atlantic, where the Gulf Stream releases its heat, it warms Europe’s climate as much as if the continent’s sunlight were increased by a third.

As the waters of the Gulf Stream yield their heat they sink, forming a great mid-oceanic waterfall. This waterfall is the powerhouse of the ocean currents of the entire planet, but history shows us that it has been interrupted many times.

Fresh water disrupts the Gulf Stream because it dilutes its saltiness, preventing it from sinking and thus disrupting the circulation of the oceans worldwide. Several Sverdrups or more of freshwater flow is needed. If the frozen north melted it could achieve that liquid potential, and to this we must add the increasing rainfall across the region.

The tropical Atlantic is becoming saltier at all depths, while the North and South polar Atlantic are becoming fresher. The change is due to increased evaporation near the equator and enhanced rainfall near the Poles. When similar changes were observed in other oceans, scientists realised that something—most probably climate change—had accelerated evaporation and rainfall rates over the oceans by 5 to 10 per cent.

This increasing tropical saltiness could lead to a temporary quickening of the Gulf Stream before its shutdown. Extra heat would be transferred to the Poles, which would melt more ice until enough fresh water could flow into the North Atlantic, collapsing the system altogether.

How fast might it happen? Ice-cores from Greenland indicate that, as the Gulf Stream slowed in the past, the island experienced a massive 10°C drop in temperature in as little as a decade. Presumably, similarly rapid changes were also felt over Europe, although no detailed record of climate has survived to tell of it.

It is possible, if the Gulf Stream were to slow, that extreme falls in temperature could be felt over Europe and North America within a couple of winters.

When is such an event likely to happen? Some climatologists think they are already seeing early signs of a shutdown. Not all agree—scientists at the Hadley Centre in England rate the chance of major disruption to the Gulf Stream this century at 5 per cent or less. Their main concern is an event in the Amazon that could be even more catastrophic.

SCENARIO 2
Collapse of the Amazon rainforests

One of the Hadley Centre’s computer models is known as TRIFFID (Top-down Representation of Interactive Foliage and Flora Including Dynamics). It suggests that, as the concentration of CO₂ in the atmosphere increases, plants— particularly in the Amazon—start behaving in unusual ways.

The plants of the Amazon effectively create their own rainfall—the volume of water they transpire is so vast that it forms clouds whose moisture falls as rain, only to be transpired again and again.

But CO₂ does odd things to plant transpiration. Plants, of course, generally don’t wish to lose their water vapour, as they have gone to some trouble to convey it from their roots to their leaves. Inevitably they do lose some whenever they open the breathing holes (stomata) in their leaves. They open their stomata to gain CO₂ from the atmosphere, and they will keep them open only as long as required.

Thus, as CO₂ levels increase, the plants of the Amazonian rainforest will keep their stomata open for briefer periods, and transpiration will be reduced. And with less transpiration there will be less rain.

TRIFFID indicates that, by around 2100, levels of CO₂ will have increased to the point that Amazonian rainfall will reduce dramatically, with 20 per cent of that decline attributable to closed stomata. The rest of the decline, the model predicts, will be due to a persistent drought that will develop as our globe warms.

The current average rainfall in the Amazon of 5 millimetres per day will decline to 2 millimetres per day by 2100, while in the northeast it will fall to almost zero. These conditions, combined with an average rise in temperature of 5.5°C, will make the collapse of the Amazonian rainforest inevitable. A small change in temperature is capable of turning soils from absorbers of CO₂ to large-scale producers. As the soil warms, decomposition accelerates and lots of CO₂ is released. This is a classic example of a positive feedback, where increasing temperature leads directly to a vast increase in CO₂ in the atmosphere, which further increases temperature. With the loss of the rainforest canopy, soils would heat and decompose more rapidly, which would lead to the release of yet more CO₂.

This means a massive disruption of the carbon cycle. The storage of carbon in living vegetation would fall by 35 gigatonnes, and in soil by 150 gigatonnes. These are huge figures—totalling around 8 per cent of all carbon stored in the world’s vegetation and soils!

The outcome of this series of positive feedback loops is that by 2100 the Earth’s atmosphere would have close to 1000 parts per million of CO₂. Remember our current level is 380 parts per million and we need to act now to stop it reaching 550 parts per million.

This modelling experiment predicts devastation in the Amazon Basin. Temperatures rise by 10°C. Most of the tree-cover is replaced by grasses, shrubs, or at best a savannah studded with the odd tree. Large areas become so hot and blighted that they cannot support even this reduced vegetation, and turn into barren desert.

When might all of this happen? If the model is correct we would start to see signs of Amazonian rainforest collapse around 2040.

By the end of the century the process would be complete. Half of the deforested region will turn to grass, the other half to desert.

What is so terrifying about this scenario is that climate change in the Amazon would itself hasten further runaway global climate change.

SCENARIO 3
Methane release from the sea floor

Clathrates is the Latin word for ‘caged’ and the name refers to the way ice crystals trap molecules of methane. Clathrates are also known as the ‘ice that burns’. They contain lots of gas under high pressure, which is why pieces of the icy substance hiss, pop and, if lighted, burn when brought to the surface.

Massive volumes of clathrates lie buried in the seabed right round the world—perhaps twice as much in energy terms as all other fossil fuels combined. The clathrates in the seabed are kept solid only by the pressure of the cold overlying water. There are masses of clathrates in the Arctic Ocean, where temperatures are sufficiently low, even near the surface, to keep them stable.

It’s illustrative of the endless ingenuity of life that some marine worms survive by feeding on the methane in clathrates. They live in burrows within the icy matrix, which they mine for their energy requirements. There are between 10,000 and 42,000 trillion cubic metres of the stuff scattered around the ocean floor, compared with the 368 trillion cubic metres of recoverable natural gas in the world. It’s not surprising that both worms and the fossil fuel industry can see a future in this weird material.

If pressure on the clathrates were ever relieved, or the temperature of the deep or Arctic oceans were to increase, colossal amounts of methane could be released. Palaeontologists are now beginning to suspect that the unleashing of the clathrates may have been responsible for the biggest extinction event of all time 245 million years ago.

At that time around nine out of ten species living on Earth became extinct. Known as the Permo-Triassic extinction event, it destroyed early mammal-like creatures, thus opening the way for the dominance of the dinosaurs.

Many people think the cause of the extinction may have been a massive outpouring of lava, CO₂ and sulphur dioxide from the Siberian Trap volcanoes (the largest known flood basalt area). This would have led to an initial rise in global average temperature of about 6°C and widespread acid rain, which would have released yet more carbon. The increasing temperature then triggered the release of huge volumes of methane from the tundra and from clathrates on the sea floor. The explosive power to change climate would have been beyond imagination.

Two of these scenarios—the Amazonian die-back and the release of the clathrates—involve positive feedback loops, where changes build on each other to produce even greater changes. But there’s one other positive feedback loop that’s already occurring, and may be the trigger for further change.

Throughout our history we have engaged in a constant battle to maintain a comfortable body temperature, which has been very costly in terms of time and energy. Just think of the hundreds of slight shifts in body position we make every day and night, and the taking off and putting on of overcoats and hats. Purchasing a house, our greatest personal expense, is primarily about regulating our local climate. In the US, 55 per cent of the total domestic energy budget is devoted to home heating and air conditioning. Home heating alone costs Americans US$44 billion per year.

As our world becomes more uncomfortable because of climate change, the demand for air conditioning will increase. In fact, during heatwaves it could mean the difference between life and death. But, unless we change how we create electricity, that demand for air conditioning will be met by burning more fossil fuels, which is a powerful positive feedback loop.

As global warming speeds up we will huddle at home clutching the remote of our climate control system, releasing ever more greenhouse gases. There is already a huge demand for air conditioners in countries such as the US and Australia where, until recently, construction codes for houses have been appallingly lax in regard to energy use.

Will we, in order to cool our homes, end up cooking our planet? Will air conditioning be one of the causes of the collapse of the Amazon or the interruption of the Gulf Stream?