The Stern Review: The Science of Climate Change

The world is warming up. We face ecological, social and economic meltdown, famine, drought, disease and turf wars. For real?

For years, scientists have been grappling with climate model after climate model, trying to fully understand the causes, and predict the progression of what has come to be known as global climatic change. In 2001, the Intergovernmental Panel on Climate Change (IPCC) provided irrefutable evidence that the overriding cause of climate change is the human-induced increase of greenhouse gases in our atmosphere. Ever since, as models have increased in their sophistication and complexity, so the true horrors of what climate change holds for our future have been revealed.

What generates our climate?

‘Climate’, simply described, is the average weather over a certain period of time and space. With recorded surface temperatures today ranging from +58°C to -89°C, Earth’s regional climates are highly variable from equator to poles, and across its latitudes. These great variations are brought about by the uneven fall of the Sun’s parallel rays across the Earth’s curved surface, which heat it more at the equator than at the poles, as well as the lack of physical uniformity of our planet, with land, water and ice distributed non-symmetrically across its surface, each with differing reflective properties.

These two factors influence the way that heat is distributed around the globe, and bring about large-scale movements of air and water, which in turn dictate local patterns of rainfall and nutrient availability. This regional climatic diversity has given rise to a great plethora of life forms, all adapted to their own particular conditions with incredible finesse. There are rainforests, deserts, savannah, coral reefs and mangroves in the tropics; deciduous forests, rich grasslands, chaparral, salt marshes and kelp forests in more temperate climes; taiga, tundra and alpine biomes towards the poles and dizzy heights, and icy wilds at the poles themselves.

Yet Earth’s global climate – the average of all its climatic regions – is something quite different. This is far less variable, changing only over a scale of thousands to millions of years. Looking back into Earth’s 4.6-billion-year history, it has fluctuated between a ‘snowball Earth’ and steamy hothouse, with many stages in between, and with these changes there have occurred both great mass extinctions and explosive innovations, shaping life’s winding evolutionary path. In this context, modern Man, who finally appeared about 200,000 years ago, has existed but a moment in evolutionary time.

Determined by the relationship between the amount of energy received and lost from its system, Earth’s global climate depends on the amount of energy received from the Sun, and the amount of energy trapped at its surface. Although simple in principle, these basic parameters are influenced by a host of factors, which operate on a wide range of temporal and geographical scales, and interact with one another. The amount of solar energy received by the Earth – the energy input – is affected by fluctuations in solar activity, small variations in Earth’s orbit around the Sun (Milankovitch cycles), and any processes that influence the amount of particulate matter in the Earth’s atmosphere (which can reduce the amount of solar energy reaching the Earth), such as volcanic activity, asteroid impact or, most recently, flying at high altitudes.

The amount of energy trapped at the Earth’s surface is determined by a special property of its atmosphere. The Earth’s atmosphere is made up of a number of gases, each with different properties, which are held close to its surface by the forces of gravity. Some, known as the greenhouse gases (GHGs), have a particular structure that enables them to absorb and re-emit heat. These include water vapour (H2O), carbon dioxide (

CO2), methane ( CH4), nitrous oxide ( N2O), ozone ( O3) and the halocarbons (eg CFCs), and they perform a crucial function in maintaining the Earth’s climate by trapping heat at its surface. They all have different strengths; methane, for example, having 23 times the ability to trap heat compared to CO2, and nitrous oxide 296 times.

The presence of GHGs in the atmosphere is essential to life on Earth, allowing it to maintain an average surface temperature between the boiling and freezing point of water. If they did not exist, the Earth’s surface would be 30°C cooler, and similar to the moon.

As the Sun’s rays, made up primarily of visible light, reach the Earth’s surface, some are absorbed, and some reflected, dependent on what they meet. Those that are absorbed are re-emitted as infra-red radiation (heat). The loosely bound atomic formation of GHG molecules causes them to absorb this reflected heat energy, which they then re-radiate in all directions, where it is likely to be reabsorbed by other GHG molecules. This process of absorption, emission and reabsorption – now known as the Greenhouse Effect – traps heat energy, like a planetary duvet, significantly reducing the amount of heat that is re-radiated out to space. GHGs are kept in a delicate balance in the atmosphere by the natural elemental cycles – for example the carbon and nitrogen cycles – that take place between all living organisms and their environment.

Consequently our climate is mediated not only by numerous physical forces, both external and internal to the Earth, but also by multifaceted life itself. This multitude of interactions is what makes climate science and climate predictions so very complex and so very sensitive.

What is causing climate change?

It is now beyond any reasonable doubt that climate change is happening, it’s happening fast, and it is caused by the human input of GHGs into the atmosphere. It is true that the Earth’s climate has changed in the past. However, these changes were either slow enough to allow life to move on and adapt to the conditions, or they brought about large-scale extinctions.

The past century has seen a massive shift upwards in average global temperature, to levels not experienced in the previous 650,000 years of Earth’s history, and which are almost certainly beyond the evolutionary experience of modern Man. Global temperatures have risen by 0.7°C in the last century, and they are still rising (with a commitment to 1.6°C already, due to a time lag in the effects of GHGs). This temperature rise tightly correlates with increasing concentrations of GHGs in our atmosphere.

Atmospheric levels of CO2 have been constant, at around 280 parts per million (ppm) for the last 1000 years, but since the dawn of the industrial revolution, in the early 1800s, they have risen by a third, to 380ppm today. CO2 is not the only GHG that has increased in atmospheric concentration either. Others, particularly methane and nitrous oxide have also increased. Methane emissions currently account for around 15 per cent of all GHG emissions, but are predicted to increase to 50 per cent by 2100. Thus the warming effect is far greater than that simply produced by CO2 and is currently equivalent to 430 parts per million (ppm) CO2e (ie CO2equivalent).

This rate of increase is unprecedented during at least the past 20,000 years, according to IPCC scientists, and the present CO2 concentration has not been exceeded during the past 420,000 years and is likely (66-90 per cent chance) not to have been during the past 20 million years.

As was firmly established in the 2001 IPPC report, and is supported by Stern’s recent review, this increase in atmospheric GHG concentration is the result of human activity. Crucially, Stern notes that while natural factors (such as orbital variation and solar activity) could have explained some of the early-19th-century trends, we would then have expected a slight cooling over the past 50 years if they were due to natural factors alone, and this strongly contrasts with the observed rise.

It is therefore irrefutable that this buildup of GHGs in our atmosphere is driving global climate change, and that this is due to human activity.

This accumulation of GHGs is primarily driven by the burning of fossil fuels, to serve the ever-growing energy needs of the human population. As the sheer scale of the human enterprise grows, as we con continue to industrialise, and the world becomes an ever more global economy, GHG emissions keep on rising. Fossil fuel burning, however, is not the only source of emissions. Deforestation, current farming practices, mining, and poor waste management are also key contributors.

What are the implications?

For the past 8,000 years, our planet has been enjoying a remarkably stable climate. This has allowed agriculture to commence, cities to swell, cultures to flourish and hence human civilisation to blossom. Prior to this period of climatic stability, man existed, for the previous 140,000 years, as a hunter-gatherer, unable to settle anywhere for long in a fluctuating and harsh climate. We take it for granted, yet our reliance on a stable climate is fundamental to the structure of our modern society.

Our climate has already warmed by 0.7ºC above pre-industrial levels, and even with this seemingly small rise we can see significant changes occurring across the planet. Earth’s polar ice caps and land glaciers are melting, which along with the thermal expansion of water has contributed to a sea level rise of 0.2m; worldwide weather patterns are becoming increasingly unpredictable and violent, Hurricane Katrina wreaking havoc in New Orleans in 2005; heatwaves, droughts and flooding are ever more frequent, with the 2003 heatwave in Europe alone claiming an estimated 35,000 lives; and ecosystems are struggling, as the wilting coral reefs in the Indian and Pacific Oceans serve to remind us. These changes, however, are a mere whisper of what may come if we do not act swiftly and decisively.

At current projections, the IPCC predict atmospheric GHG concentrations to rise to 550-700ppm CO2e by 2050, and 650- 1,200ppm by 1200, which will mean a global climatic warming of 2.5°C or more by 2050. It is crucial to note that these rises in temperature won’t be uniform, with some regions – primarily the polar regions and midlatitudes – experiencing far more severe levels of warming. Furthermore, as Stern points out, these figures assume that emissions stay at current levels. As they are currently increasing every year, these are highly conservative estimates.

Yet even with this degree of warming, the impacts will be enormous (see fig, p18). Sea levels are expected to rise by more than 0.5 metres, affecting tens to hundreds of millions of people each year. The water cycle will further intensify, increasing rainfall and flooding in some areas, and droughts and heatwaves in others. Some estimates suggest that these changes, in combination with rising sea levels, will result in 200 million people becoming permanently displaced, and up to four billion people experiencing growing water shortages. Furthermore, melting glaciers will increase flood risk during the wet season, and strongly reduce dry season water supplies to one-sixth of the world’s population.

These changes to water supplies, in combination with higher temperatures, will result in declining crop yields, which are likely to leave hundreds of millions, par particularly in Africa, without the ability to produce or purchase sufficient food. Meanwhile, continued ocean acidification – a direct result of rising CO2 levels – will have major effects on marine ecosystems, with possibly adverse consequences on fish stocks, which are already pushed to the brink of collapse by overfishing.

In terms of human health, not only will there be an increase in worldwide deaths from malnutrition and heat stress; vector-borne diseases, such as malaria and dengue fever, could become more widespread as temperatures rise in the temperate zones.

Already, the number of extreme weather events of all kinds has quintupled since the 1950s, and the frequency of very intense hurricanes and typhoons has doubled since the 1970s. Continued climate change is likely to increase the intensity of storms further. Peak wind speeds of tropical storms are a strongly exponential function of temperature, increasing by 15-20 per cent for a 3°C increase in tropical sea surface temperatures. Storms and associated flooding are already the most costly natural disaster today, making up 90 per cent of total losses from natural catastrophes in 2005.

However, perhaps the least well known, yet most important, impact of such levels of climatic change, are its effects on our struggling global eco-systems. As the climate changes, we are losing an ever-increasing degree of biodiversity, as species flail under the strains of foreign climates that they are not adapted to weather. One study estimates that with only 2°C of warming, around 15-40 per cent of species face extinction, as climate change is occurring too fast for species to adapt.

Apart from being a source of innumerable direct and irreplaceable benefits such as food, medicine, materials, recreation and information, biodiversity both underpins and supports a wealth of life-sustaining processes that are provided by ecosystems across the globe.

These include the regulation of our global climate and atmosphere, the cycling of nutrients, the purification and retention of fresh water, and the formation and enrichment of soil, to name but a few. These services have been conservatively valued at two times global GDP, the most horrifying market externality yet. Such largescale losses of biodiversity put the continued functioning of Earth’s eco-systems under great threat, diminishing both their resilience and efficiency. Crucially, we have neither the technology nor the funds to replace the services they provide, and, as they become increasingly ragged, Earth becomes ever more vulnerable to change.

Even more disturbing, and so poorly understood that it has been omitted from most climatic models, is the danger of so-called ‘positive feedbacks’ (or ‘non-linear’ changes in the climate). Because of the interconnected nature of the climate system, any changes to one aspect ricochet through it and affect many more. These feedbacks can have either a warming or cooling effect on the climate trend. As our climate warms, there is an increasing risk that natural processes could be set in motion that could drive atmospheric GHG concentrations far higher than human activity alone ever would, either by reducing natural absorption of CO2 or releasing stores of CO2 and methane.

These processes include weakening the ability of natural carbon sinks, such as the forests and oceans, to absorb CO2 from the atmosphere; a reduction of forest biomass through drought, which would release tonnes of CO2 into the atmosphere and leave it vulnerable to forest fires (as in Indonesia in a poor el Niño year) the thawing of Arctic permafrost and warming of wetlands, which hold (in the form of methane and CO2) more than double the total cumulative emissions of fossil fuels so far; the release of huge quantities of methane from its cold hydrate stores in the ocean floor; and a reduction in the reflectivity of the polar regions (albedo) as polar ice melts.

Worryingly, there are signs that these changes are already occurring. Substantial thawing of permafrost has begun in some areas, and methane emissions in Northern Siberia have increased by 60 per cent since the mid 1970s. The Amazon is suffering the worst drought in more than a century, and the Met Office has warned that by 2030, the total carrying capacity of the biosphere to absorb carbon will have reduced from the current 4 billion tonnes a year, to 2.7 billion.

It is predicted that these climatic feedbacks could drive Earth’s climate up by a staggering 10°C by 2100. This would melt the Antarctic and Greenland ice sheets, causing rises of up to 12m in sea levels over centuries or millennia, would slow or stop the North Atlantic Thermohaline Circulation (from which the Gulf Stream arises), throwing a large part of the Northern Hemisphere into Siberian weather conditions and, perhaps most crucially, would spell catastrophe for the various biota across the planet upon which we rely for our oxygen, water, food and materials. In no uncertain terms, such changes would precipitate the sixth mass extinction of life that our planet has witnessed.

It is a sobering thought that such runaway processes are known to have precipitated mass extinctions in the past. The end-Permian extinction, 251 million years ago, involved a 6°C global temperature rise, thought to have been precipitated by climatic feedbacks following an initial volcanic eruption or meteorite impact. It resulted in a loss of as much as 95 per cent of the species on the planet, almost bringing an end to life itself. As Stern notes in his report, a warming of 5°C on a global scale would be far outside the experience of human civilisation and comparable to the difference between temperatures during the last ice age and today.

Scientists have predicted that a rise of more than 2°C is the point at which some of the most dangerous runaway processes could become irreversible. Furthermore, given that climatic predictions are highly conservative, we must err on the side of caution as to what will be safe CO2 concentrations to reach. As the current atmospheric concentrations of GHGs mean we are already committed to a rise of 1.6°C, global action needs to be rapid, bold and unified.

So what can be done?

Data from 11 separate studies indicates that, stabilising at 450ppm CO2e, the probability of exceeding a 2°C rise, relative to pre-industrial levels, is 26-78 per cent. This probability sharply increases from this point, to 63-99 per cent with stabilisation at 550ppm CO2e.

We have already passed 400ppm CO2e (we are now at 430ppm CO2e), and stabilising at 450ppm CO2e, without overshooting, will require immediate and substantial cuts. Overshooting this level carries many inherent dangers, from which Stern warns it may not be possible to recover. According to Stern, in order to meet this target, global emissions would need to peak in the next 10 years, and then fall at more than 5 per cent per year, reaching 70 per cent below current levels by 2050. Yet if global citizens are all to be entitled to an equal emissions quota, reaching this level requires cuts of 87 per cent for UK citizens. However, emission levels are still rising.

Climate science plainly states that the degree of warming that occurs profoundly affects the impacts we experience. Every small incremental rise in temperature bears the burden of many more deaths and suffering, and carries an ever-increasing threat of further change. Although significant challenges in the process of climate modelling clearly remain, the evidence is strong enough to dictate that any rise above 2°C should be avoided at all costs. Climate change is the greatest threat mankind has ever faced. Every irresponsible decision now made amounts to genocide.

In an evolutionary context, our society and our economy are mere spiders’ threads against the blowing gales of the elemental forces. We rely – as does all life – on air, water, food, and warmth to survive; and without these fundamental things, our mirage of social constructs will turn to dust. We are at the mercy of the elements, as we always have been.

This article first appeared in the Ecologist December 2006

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