Chapter 1
What is climate change?
Future climate change is one of the defining challenges of the 21st century, along with global inequality, environmental degradation, and global insecurity. The problem is that ‘climate change’ is no longer just a scientific concern, but encompasses economics, sociology, geopolitics, national and local politics, law, and health, just to name a few. This chapter will examine the role of greenhouse gases (GHGs) in moderating past global climate, why they have been rising since the Industrial Revolution, and why they are now considered dangerous pollutants. It will examine which countries have produced the most anthropogenic GHGs and how this is changing with rapid economic development. It will introduce the Intergovernmental Panel on Climate Change (IPCC) and how it regularly collates and assesses the most recent evidence for climate change.
The Earth’s natural greenhouse
The temperature of the Earth is determined by the balance between energy received from the Sun and its loss back into space. The Sun’s energy consists of short-wave radiation (mainly visible ‘light’ and ultraviolet (UV) radiation) and nearly all of it passes through the atmosphere without interference (see Figure 2). The only exception is damaging high-energy UV, which is absorbed by atmospheric ozone. About one-third of the solar energy is reflected straight back into space. The remaining energy is absorbed by the surface of the Earth. This energy warms the land and the oceans, and this heat is radiated back as long-wave infrared or ‘heat’ radiation.
2. The greenhouse effect. Greenhouse gases trap some of the Earth’s heat before releasing it to warm the atmosphere.
Atmospheric gases such as water vapour, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are known as GHGs as they absorb some of this long-wave radiation, warming the atmosphere. This effect has been measured in the atmosphere and can be reproduced time and time again in the laboratory. Without this natural greenhouse effect the Earth would be at least 35°Celsius (C) colder, making the average temperature in the tropics about −10°C. Since the Industrial Revolution, we have been burning fossil fuels (oil, coal, and natural gas) deposited hundreds of millions of years ago, releasing the carbon back into the atmosphere as CO2 and CH4, increasing the ‘greenhouse effect’, and elevating the temperature of the Earth. In effect we are burning fossilized sunlight.
Past climate
Climate change in the geological past has been reconstructed using a number of key archives, including marine and lake sediments, ice cores, cave deposits, and tree rings. These various records reveal that over the past 50 million years the Earth’s climate has been cooling down, moving from the so-called ‘greenhouse world’ of the Eocene, with warm and gentle conditions, through to the cooler and more dynamic ‘ice house world’ of today. It may seem odd that in geological terms our planet is extremely cold, while this whole book is concerned with our rapid warming of the planet. This is because the very fact that there are huge ice sheets on both Antarctica and Greenland, and nearly permanent sea ice in the Arctic Ocean, makes the global climate very sensitive to changes in GHGs.
The long-term global cooling of the Earth kicked off with the glaciation of Antarctica about 35 million years ago and then accelerated with the great Northern Hemisphere ice ages, which began 2.5 million years ago. Since the beginning of the great ice ages, the global climate has cycled between conditions that were similar or even slightly warmer than today, to full glacial phases, during which ice sheets over 3 kilometres (km) thick formed over much of North America and Europe. Between 2.5 and 1 million years ago, these glacial–interglacial cycles occurred every 41,000 years, and since 1 million years ago they have occurred every 100,000 years.
These great ice-age cycles are driven primarily by changes in the Earth’s orbit with respect to the Sun. In fact, the world has spent over 80% of the past 2.5 million years in conditions colder than the present. Our present interglacial, the Holocene Period, began about 10,000 years ago, and is an example of the brief warm conditions that occur between each ice age. The Holocene began with the rapid and dramatic end of the last ice age: in less than 4,000 years global temperatures increased by 6°C, global sea level rose by 120 metres (m), atmospheric CO2 increased by one-third, and atmospheric CH4 doubled.
Still, this is much slower than the changes we are seeing today. James Lovelock, in his book The Ages of Gaia, suggests that interglacials like the Holocene are the fevered state of our planet, which clearly over the past 2.5 million years prefers a colder average global temperature. Lovelock sees global warming as humanity adding to the already fevered state of the planet. These large-scale past changes in global climate are discussed in more detail in Climate: A Very Short Introduction.
Past variations in carbon dioxide
One of the pieces of scientific evidence that shows that atmospheric CO2 is an important control of global climate comes from the study of past climate. Evidence for past variations in GHGs and temperature comes from ice cores drilled in both Antarctica and Greenland. As snow falls, it is light and fluffy and contains a lot of air. As more snow falls the older snow is slowly compacted to form ice, with some of the air bubbles becoming trapped. By extracting the air from these bubbles trapped in the ancient ice, scientists can measure the percentage of GHGs that were present in the past atmosphere. Scientists have drilled down over 2 miles into both the Greenland and the Antarctic ice sheets, which has enabled them to reconstruct the amount of GHGs present in the atmosphere over the past million years. By examining the oxygen and hydrogen isotopes in the frozen water that make up the ice core, it is possible to estimate the air temperature above the ice sheet when the water first froze.
The results are striking: the proportion of GHGs such as atmospheric CO2 and CH4 co-vary with temperature over the past 800,000 years (see Figure 3). The cyclic changes in climate from glacial to interglacial periods can be seen both in temperatures and the GHG content of the atmosphere. This strongly supports the idea that GHGs in the atmosphere and global temperature are closely linked; when CO2 and CH4 increase, global temperatures increase, and vice versa when they decrease.
3. Greenhouse gases and temperature for the past eight glacial cycles recorded in ice cores.
Early farmers
The high-resolution ice-core evidence from Greenland and the continental margins of Antarctica shows that GHGs in the atmosphere rose a small amount before the Industrial Revolution in the 1700s. Bill Ruddiman, Professor of Palaeoclimatology at the University of Virginia, suggested that early agriculturalists caused this reversal in natural decline in GHGs. Deforestation and land clearing caused the rise in atmospheric CO2 starting about 7,000 years ago, while the expansion of wet rice agriculture and cattle caused atmospheric CH4 to start rising about 5,000 years ago. It seems that early human interactions with our environment increased atmospheric GHGs just enough that even prior to the Industrial Revolution we had already delayed the onset of the next ice age, which would otherwise have started gently to occur any time in the next 1,000 years.
The Industrial Revolution
There is clear evidence that levels of atmospheric CO2 have been rising ever since the beginning of the Industrial Revolution. The first measurements of CO2 concentrations in the atmosphere started in 1958, on the summit of Mauna Loa Mountain in Hawaii at an altitude of about 4,000 m. The measurements were made at this remote location to avoid contamination from local pollution sources. The record clearly shows that atmospheric concentrations of CO2 have increased every single year since 1958. The mean concentration of approximately 316 parts per million by volume (ppmv) in 1958 has risen to over 420 ppmv today (see Figure 4). The annual variations in the Mauna Loa observatory are mostly due to CO2 uptake by growing plants. The uptake is highest in Northern Hemisphere springtime due to the great expanse of land and hence every spring there is a drop in atmospheric CO2, which unfortunately does nothing to change the overall trend towards ever higher values.
4. Mauna Loa observatory atmospheric carbon dioxide measurements.
The Mauna Loa observatory CO2 data can be combined with the detailed ice-core evidence to produce a complete record of atmospheric CO2 since the beginning of the Industrial Revolution. This shows that atmospheric CO2 has increased from a pre-industrial concentration of about 280 ppmv to over 420 ppmv at present, representing an increase of over 45%. To put this increase into context, ice-core evidence shows that over the last 800,000 years the natural change in atmospheric CO2 has been between about 200 and 280 ppmv. The variation between warm and cold periods is about 80 ppmv—less than the CO2 pollution that we have put into the atmosphere over the past 100 years. The level of human pollution caused in one century has been greater than natural variations, which took thousands of years.
Who produces the pollution?
The United Nations Framework Convention on Climate Change (UNFCCC) was created to produce the first international agreement on reducing global GHG emissions. This is not a simple task as CO2 emissions are not evenly produced by countries. According to the IPCC (see Box 1) the primary source of CO2 is the burning of fossil fuels: over 85% of the global CO2 emissions comes from energy production, industrial processes, and transport. These emissions are not evenly distributed around the world because of the unequal distribution of industry and wealth: North America, Europe, and Asia emit over 90% of the global, industrially produced CO2 (see Figure 5). Moreover, historically, the developed nations have emitted much more than less developed countries.
Box 1 What is the IPCC?
The IPCC was established in 1988 jointly by the United Nations Environmental Panel and the World Meteorological Organization to address concerns about global warming. The purpose of the IPCC is the continued assessment of the state of knowledge on the various aspects of climate change, including scientific, environmental, and socioeconomic impacts and response strategies. The IPCC does not undertake independent scientific research, rather it brings together all key research published in the world and produces a consensus. There have been six main IPCC reports—in 1990, 1996, 2001, 2007, 2013/14, and 2021/2—and many individual, specialized reports on such subjects as carbon-emission scenarios, alternative energy sources, oceans, land use, and extreme weather events.
The IPCC is recognized as the most authoritative scientific and technical voice on climate change, and its assessments have had a profound influence on the negotiators of the UNFCCC. The IPCC is organized into three working groups plus a task force to calculate the amount of GHGs produced by each country. Each of these four bodies has two co-chairs (one from a developed and one from a developing country) and a technical support unit. Working Group I assesses the scientific aspects of the climate system and climate change; Working Group II addresses the vulnerability of human and natural systems to climate change, and options for adapting to climate change; and Working Group III assesses options for limiting GHGs emissions and otherwise mitigating climate change.
The IPCC provides governments with scientific, technical, and socioeconomic information relevant to evaluating the risks and to developing a response to global climate change. The latest reports from these three working groups were published in 2021—with approximately 500 experts, from some 120 countries, directly involved in drafting, revising, and finalizing the IPCC reports, as well as thousands of additional experts participating in the review process. The IPCC authors are always nominated by governments and international organizations, including non-governmental organizations (NGOs). These reports are essential reading for anyone interested in climate change and are listed in the Further Reading section at the end of this book. In 2008, the IPCC was jointly awarded, with Al Gore, the Nobel Peace Prize, to acknowledge all the work the IPCC had done over the previous 20 years.
5. Historic carbon dioxide emissions by region.
The second major source, accounting for 10‒15% of global CO2 emissions, is land-use changes. These emissions come primarily from the cutting down of forests for the purposes of agriculture, urbanization, or building roads. When rainforest is cut down the land often turns into less productive grassland with considerably reduced capacity for storing CO2. Here the pattern of CO2 emissions is different, with South America, Asia, and Africa being responsible for over 90% of present-day land-use change emissions. This raises important ethical questions because it is difficult to tell these countries to stop deforestation when this has already occurred in much of North America and Europe before the beginning of the 20th century. In terms of the amount of CO2 released, industrial processes still significantly outweigh land-use changes.
We have put nearly half a trillion tonnes of carbon into the atmosphere since the Industrial Revolution, but this still amounts to only half of our total emissions. The other half has been absorbed by the Earth—with 25% going into the oceans and 25% going into the land biosphere. Scientists are concerned that this uptake of our pollution is unlikely to continue at the same level in the future. This is because as global temperatures rise the oceans will warm and will be able to hold less dissolved CO2. As we continue to deforest and convert land for farming and urbanization, there will be less vegetation to absorb CO2, again reducing the uptake of our carbon pollution (Figure 6).
6. Historic global sinks and sources of carbon dioxide.
There is clear evidence that GHG concentrations in the atmosphere have been rising since the Industrial Revolution of the 18th century. Atmospheric concentrations of both CO2 and CH4 are higher now than at any time within at least the past 3 million years. Within 100 years, we have put more than one and a half times the amount of carbon into the atmosphere than was emitted over the 4,000 years’ transition from the last ice age to our current interglacial period.
The current scientific consensus is that these recent changes in GHG concentrations in the atmosphere have already caused global temperatures to increase. Since 1880, the global average surface temperature has increased by 1.1°C. This warming has been accompanied by a significant warming of the ocean, a rise in sea level of over 24 centimetres (cm), a 50% decline in Arctic sea ice, and an increase in the number of extreme weather events. As we emit more and more carbon into the atmosphere, the effects in terms of climate change will increasingly threaten and challenge human society.
The science, politics, and potential solutions to climate change are examined in the rest of this book. In Chapter 2 the emergence of climate change as a global issue is discussed. Chapters 3 and 4 consider the current scientific evidence for climate change, and how scientists are modelling the future to assess the ways in which global carbon emissions will alter our climate. Chapters 5 and 6 examine the impacts of these future climate changes and the possibility that there may be hidden surprises within the climate system that may exacerbate climate change. Chapters 7 and 8 investigate the political aspects of climate change, and the potential political, economic, and technological solutions available to us. Finally, Chapter 9 provides multiple views of the future, dependent on our future carbon emissions, and discusses how we could resolve the climate change crisis.