Preface

The debate about human-induced climate change may still be raging in certain sections of the media, but here we start from a position of acceptance: acceptance that increased greenhouse gas (GHG) concentrations in the atmosphere are leading to increased global temperatures, and that human activities are playing an ever-growing role in this increase.

To briefly set the scene, let us look at the evidence for human influence on global climate. The 'greenhouse effect' is essentially the trapping of infrared radiation from the earth's surface by 'greenhouse gases' (though greenhouses warm up primarily through the glass keeping heat in, rather than through GHGs trapping the heat). We actually need GHGs in our atmosphere; without them the average temperature on earth would be around -18°C. Unfortunately, in the last 200 years or so the activities of humans have pumped a lot of extra GHG into the atmosphere and this has led to an 'enhanced greenhouse effect' and thereby to the extra global warming we are now experiencing.

By increasing the atmosphere's ability to absorb infrared energy, our greenhouse emissions are disturbing the way the climate maintains the balance between incoming and outgoing energy. A doubling of the concentration of GHGs (predicted in the next 100 years) would, if nothing else changed, reduce the rate at which the planet can shed energy into space by about 2%. The climate will somehow have to adjust - by heating up - and while 2% does not seem much, across the entire earth it is equivalent to trapping the energy content of about 3 million tonnes of oil every 10 min.

Long-term data-sets for global temperature indicate a clear and consistent increase in global temperatures, particularly since the industrial revolution. Much of the argument surrounding climate change hinges not on whether global warming is occurring, but rather whether this warming is a result of human activity. Natural drivers of global climate, such as solar and volcanic activity, have been invoked by some to explain the accelerated rate of global warming observed in recent decades. However, such natural drivers of global temperatures cannot explain the warming that has occurred since 1840. The prime culprit for the observed increase: elevated GHG concentrations.

As concentrations of the main GHGs have risen in our atmosphere, global temperatures have also increased. A rapid increase in atmospheric concentrations of the three main anthropogenic (human-made) GHGs - carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) - is clear from measurements over the past few decades. Ice-core records for these gases show that their concentrations in the atmosphere are now higher than at any time in the last 650,000 years, and probably even in the last 20 million years.

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Fig. P.1. Global atmospheric concentrations of three well-mixed greenhouse gases. (From the Intergovernmental Panel on Climate Change (IPCC). Reproduced with permission.)

Since around the time of the industrial revolution, levels of CO2, CH4 and N2O have all risen dramatically. Fossil fuel combustion, increasingly intensive agriculture and an expanding global human population have been the primary causes for this rapid increase.

Sulphate aerosols, though not GHGs, are none the less very important to global climate. Sulphate in our atmosphere has a net cooling effect and therefore reduces the warming effect of the GHGs to a certain extent. The same increases in fossil fuel burning that have led to elevated GHG concentrations in the last 200 years have also led to an increase in sulphate emissions. Cleaner fuel technologies are now leading towards a reduction in sulphate emissions and their incidental cooling effect (commonly known as global dimming) on our climate. If GHG emissions continue to increase, their overall warming effect may become even greater.

The complex interaction of positive and negative influences (the feedbacks) on global climate, together with the uncertainty over how anthropogenic GHG emissions will change in coming decades makes predicting future warming difficult. The problem is exacerbated by our poor level of understanding of exactly how some key factors, such as albedo (the reflectance of the earth's surface) and cloud formation, operate and interact with a changing climate. What we do know is that increases in concentrations of GHGs in our atmosphere risk rapid increases in global temperature. This in turn is likely to lead to rapid changes in climate and sea level that may threaten civilization itself. Indeed, the publication of the Stern Report in 2006 highlighted the grave economic consequences of our failing to tackle human-induced global warming immediately.

It is against this backdrop that Greenhouse Gas Sinks has been written. This book brings together chapters written by leading scientists from across Europe, the USA, Canada and Australia. It represents an expert synthesis of GHG sink science and how this applies to the past, current and future changes in climate. The 'sinks' discussed here are the planet's storage areas, where GHGs are locked away from the atmosphere and thus prevented from contributing to global warming.

As will become apparent, the sinks for CO2, CH4 and N2O play a vital role in determining the concentrations of these GHGs in our atmosphere, but all are vulnerable to human activities.

The aim of this book is to provide readers with in-depth, authoritative information on these sinks. We will explore how the sinks may respond to increased GHG emissions and global temperatures, and whether we can protect and even enhance them to help mitigate climate change. These are urgent questions that must be answered as we face this greatest of challenges to humankind in the 21st century.

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