Chapter 3
Evidence for climate change
Science is not a belief system. It is a rational, logical methodology that moves forward by using detailed observation and experiments to constantly test and retest ideas and theories. It is the very foundation of our global society. So you cannot pick and choose which bits of scientific evidence you want to believe in and which bits you want to reject. For example, you cannot decide that you believe in antibiotics (as they may save your life) or that heavy metal tubes with wings can fly (because you want to go on holiday), and yet at the same time deny that smoking causes cancer, or that HIV causes AIDS, or that GHGs cause climate change. In this chapter, I present the scientific evidence that anthropogenic climate change is already happening.
Weight of evidence
If we are to understand climate change, we must understand how science works. The ‘weight of evidence’ principle prompts the constant need to compile new data and undertake new experiments in order to continually test our ideas and theories regarding climate. Over the past 40 years, the theory of climate change must have been one of the most comprehensively tested ideas in science. There are six main areas of evidence that should be considered:
In this chapter, we will consider evidence for changes in global temperature, precipitation, sea level, and extreme weather events.
Temperature
Temperatures can be estimated from a number of sources, both direct thermometer-based and proxy-based indicators. Proxy-based indicators are variables that are measured when direct measurements are not available or possible. For example, infrared (heat) satellite measurements are examples of a proxy that can be used to estimate surface temperatures.
Direct thermometer-based measurements of air temperature have been recorded at a number of sites in North America and Europe from as far back as 1760. The number of observation sites did not increase to a sufficient, worldwide geographical coverage to permit a global land average to be calculated until about the middle of the 19th century. Sea-surface temperatures (SSTs) and marine air temperatures (MATs) were systematically recorded by ships from the mid-19th century, but even today the coverage of the Southern Hemisphere is extremely poor. All these data sets require various corrections to account for changing conditions and measurement techniques. For example, for land data each station has been examined to ensure that conditions have not varied through time as a result of changes in the measurement site, instruments used, instrument shelters, or the way monthly averages were computed. We must also account for the growth of cities around some of the sites, which leads to warmer temperatures caused by the urban heat island effect. In the IPCC science report, the influence of the urban heat island effect is acknowledged as real and, if it were not corrected for, it would still be negligible for the global temperature compilation (less than 0.006°C).
For SST and MAT, there are a number of corrections that have to be applied. First, up to 1941 most SST temperature measurements were made in seawater hoisted on deck in a bucket. Since 1941, most measurements have been made at the ships’ engine water intakes. Second, between 1856 and 1910 there was a shift from wooden to canvas buckets, which changes the amount of cooling caused by evaporation that occurs as the water is being hoisted on deck. In addition, through this period there was a gradual shift from the use of sailing ships to steamships, which altered the height of the ship decks and the speed of the ships, both of which can affect the evaporative cooling of the buckets. The other key correction that has to be made is for the global distribution of meteorological stations through time, which has varied greatly since 1870.
The collation of the global temperature records has been undertaken by a number of groups around the world, including the UK Meteorological Office, National Aeronautics and Space Administration (NASA), National Oceanic and Atmospheric Administration (NOAA), and the Japan Meteorological Agency (see Figure 8). In 2012, Professor Richard Muller, a physicist and previously a climate change sceptic, and his Berkeley group collated global temperature records for the last 250 years. Because his group had not taken account of all the corrections, their estimation of global warming was higher than that of the other groups. This was subsequently revised, and Muller publicly announced he had changed his mind and that climate change was occurring and was clearly due to human activity.
By making all the necessary corrections it is possible to produce a continuous record of global surface temperature from 1880 to 2020, which shows an observed warming of between 1.0°C and 1.3°C, with 1.1°C the most likely rise over this period (Figure 8). These observations are supported by 60 years of balloon and satellite data. For example, there are over 800 stations that twice a day release rawinsondes (meteorological instruments), or balloons, to measure temperature, relative humidity, and pressure through the atmosphere to a height of about 20 km, where they burst. The temperature records also show us that the land is warming up faster than the oceans. Since 1850, the land has warmed by 1.44°C and the oceans by 0.89°C (Figure 9).
9. Land and ocean temperatures since 1850.
Global temperatures have also been reconstructed for periods of time pre-dating instrumental or thermometer records. This has been achieved by using palaeoclimate proxies such as the thickness of tree rings and the isotopic composition of ice cores or cave deposits to estimate local temperatures. Combining the GMT instrumental record with the longer palaeoclimate temperature records shows a steep rise at the end of the record and is referred to as the global warming ‘hockey stick’. A study, published in Nature in 2019, led by Raphael Neukom at the Oeschger Centre for Climate Change Research (University of Bern, Switzerland) used over 700 palaeoclimate records and showed that in the last 2,000 years, the only time the climate all around the world has changed at the same time and in the same direction has been in the last 150 years, when over 98% of the surface of the planet has warmed (see Figure 10).
10. Northern Hemisphere temperature reconstruction for the last 2,000 years.
Precipitation
There are two global precipitation data sets: Hulme and the Global Historical Climate Network (GHCN). Unfortunately, unlike temperature, rainfall and snow data are not as well documented, and recording has not been carried out for as long. It is also known that precipitation over land tends to be underestimated by up to 10–15% owing to the effects of airflow around the collecting dish. Without correction of this effect, a spurious upward trend could be perceived in global precipitation. Despite these problems there seems to be a significant increase of precipitation over the past 25 years (see Figure 11), particularly in the Northern Hemisphere middle latitudes. This is supported by evidence that since the 1980s atmospheric water content has increased over the land and ocean as well as in the upper troposphere. This is consistent with the extra water vapour that the warmer atmosphere can hold.
11. Global precipitation changes (1900–2018).
There is evidence for a global increase in precipitation but the evidence for this change is much stronger when considering individual regions. The latest IPCC report suggests that significant increases in precipitation have occurred in the eastern parts of North and South America, northern Europe, and northern and central Asia. It seems that seasonality of precipitation is also changing, for example in the high latitudes in the Northern Hemisphere, with increased rainfall in the winter and a decrease in the summer. Long-term drying trends have been observed on the Sahel, in the Mediterranean, southern Africa, and parts of southern Asia. It has also been observed that the amount of rain falling during heavy, ‘extreme’ rain events has increased.
Relative global sea level
The IPCC has also compiled all the current data on global sea level. They show that between 1901 and 2018, the global sea level rose by over 24 cm (see Figure 12). Sea-level change is difficult to measure, as relative sea-level changes have been derived from two very different data sets—tide-gauges and satellites. In the conventional tide-gauge system, the sea level is measured relative to a land-based tide-gauge benchmark. The major problem is that the land surface is much more dynamic than one would expect, with a lot of vertical movements, and these become incorporated into the measurements. Vertical movements can occur as a result of normal geological compaction of delta sediments, the withdrawal of groundwater from coastal aquifers, uplift associated with colliding tectonic plates (the most extreme of which is mountain-building such as in the Himalayas), or ongoing post-glacial rebound, and compensation elsewhere, associated with the end of the last ice age. The rebound is caused by the rapid removal of weight when the giant ice sheets melted, so that the land that has been weighed down slowly rises back to its original position. An example of this is Scotland, which is rising at a rate of 3 millimetres (mm) per year, while England is still sinking at a rate of 2 mm per year, despite the Scottish ice sheet having melted 10,000 years ago. In comparison, the problem with the satellite data is that the time covered is too short. The best satellite data started in January 1993 and show a trend of over 35 mm rise in sea level per decade. This means satellite data have to be combined with the tide-gauge data to look at long-term trends.
12. Indicators of climate change.
In summary, between 1901 and 2018 the global average sea level rose by about 2 mm per year; with the fastest rise in sea level observed between 2008 and 2018 at 4.2 mm per year. The sea-level rise of the last 30 years is made up of the following contributions: 39% from thermal expansion of the ocean; 9% Antarctic ice sheet; ~12% Greenland ice sheet; 27% glaciers and other ice caps; and another ~13% due to the overall reduced land storage of water (Figure 13). The Greenland and Antarctic ice sheets have contributed to recent sea-level rise, and this contribution is accelerating. At the moment it is estimated that Greenland is losing over 230 gigatonnes (Gt) of ice per year, a seven-fold increase since the early 1990s. Meanwhile, Antarctica is losing about 150 Gt of ice per year, a five-fold increase since the early 1990s, and most of this loss is from the northern Antarctic peninsula and the Amundsen sea sector of west Antarctica.
13. Melting of Antarctic and Greenland ice sheets.
Other evidence for global warming
Other evidence for climate change comes from the high latitudes and from monitoring extreme weather events. The annual mean Arctic sea ice extent has decreased in total between 1979 and 2018 at a rate of 3.5–4.1% per decade, which means a loss of between 0.45 and 0.51 million km2 per decade. The summer sea ice minimum has decreased even more by 12.8% per decade, which is equivalent to a loss of 1 million km2 per decade. In contrast, between 1979 and 2018 the annual mean Antarctic sea ice extent has varied markedly with record highs and lows but there is no significant trend when the continuous satellite observations for the period are examined.
There is also evidence from permafrost regions. Permafrost exists in high-latitude and high-altitude areas, where it is so cold that the ground is frozen solid to a great depth. During the summer months, only the top half-metre or so of the permafrost becomes warm enough to melt, and this is called the ‘active layer’. There has been a 3°C warming in Alaska and 2°C warming in northern European/Russian permafrost over the last 50 years, and evidence that this active layer has become much deeper. The maximum area covered by seasonal permafrost has decreased by 7% in the Northern Hemisphere since 1900, with a decrease in the spring of up to 15%. This increasingly dynamic cryosphere will amplify the natural hazards for people, structures, and communication links. Already we have seen this in the form of damage to buildings, roads, and pipelines, such as to the oil pipelines in Alaska. In addition, there is evidence that most if not all non-ice-sheet glaciers are in retreat. The amount of total snowfall and the annual snow and ice cover, particularly in the Northern Hemisphere, has greatly reduced (Figure 12). Between 1922 and 2018, over 0.27 million km2 of snow and ice cover per decade has been lost. In the Arctic, snow-cover duration has decreased on average by ~3–5 days per decade and larger declines have occurred in the Eurasian Arctic region (~12.6 days) and North American Arctic region (6.2 days).
There is also evidence that spring is occurring earlier in the Northern Hemisphere. The ice cover records from the Tornio River in Finland, which have been compiled since 1693, show that the spring thaw of the frozen river now occurs a month earlier. In Kyoto, Japan, the famous cherry blossoms now appear 21 days earlier than 100 years ago. In France, the grape harvest in Beaune is now 10 days earlier than 100 years ago. In Britain, among the variety of indicators of an earlier spring is evidence of birds nesting over 12 days earlier than 45 years ago. Insect species—including bees and termites—that need warm weather to survive are moving northward, and some have already reached England by crossing the Channel from France. Meanwhile, in the USA, species that are active in early spring such as lilac and honeysuckle are unfurling their leaves 3 weeks earlier than 40 years ago.
Extreme weather events
The latest IPCC report states that it is virtually certain that anthropogenic climate change has caused increases in the frequency and severity of hot extremes and decreases in cold extremes on most continents. The frequency and intensity of heat waves has increased in Europe, Asia, America, and Australia. The past decade has seen record-breaking heatwaves occurring in Australia, Canada, Chile, China, India, Japan, the Middle East, Pakistan, and the USA.
Climate change is also the main cause of the intensification of heavy precipitation observed over continental regions, often resulting in flooding. Record-breaking extreme floods have been recorded over the past decade in Brazil, Britain, Canada, Chile, China, East Africa, Europe, India, Indonesia, Japan, Korea, the Middle East, Nigeria, Pakistan, South Africa, Thailand, the USA, and Vietnam.
Human climate change has also played a role in shaping the global distribution and intensity of tropical cyclones. A 2020 study by James P. Kossin at NOAA and colleagues showed a 15% increase in the occurrence of the most destructive cyclones around the world over the past 40 years. Most marked was the 49% per decade increase in major hurricanes occurring in the North Atlantic and the 18% per decade increase in major cyclones in the southern Indian Ocean. In summary, the number of tropical cyclones originating in the North Atlantic Ocean, Pacific Ocean, and southern Indian Ocean has increased as well as the year-to-year variability. For example, in 2019 there were four spectacular cyclones in the Indian Ocean and two of these, in the southern Indian Ocean, were unprecedented. The 2018‒19 south-west Indian Ocean cyclone season was the costliest and most active season ever recorded since reliable records began in 1967. In 2020, Super Cyclonic Storm Amphan formed in the North Indian Ocean and made landfall in west Bengal, affecting nearly 40 million people and causing over $13 billion of damage.
The reason why scientists are certain that many of these extreme weather events are made worse by climate change is the new field of attribution science. Advances in computer processing power and improved methods for modelling the factors that contribute to weather allow scientists to run weather simulations for a region with and without anthropogenic GHG forcing. This allows us to determine the extent to which climate change has contributed to specific extreme weather events and, if there has been a contribution, whether it has increased the intensity or the frequency or both. Over 113 extreme weather events that occurred between 2015 and 2020 have been studied using attribution science: 70% of events were found to have increased frequency or intensity due to climate change; 26% were found to have a reduced occurrence due to climate change; and 4% showed no variation due to climate change.
What do the climate change deniers say?
One of the best ways to summarize the evidence for climate change is to review what the climate change deniers say against the current state-of-the-art science.
Ice-core data suggest atmospheric CO2 responds to global temperature, therefore atmospheric CO2 cannot cause global temperature changes. At the end of the last ice age the Earth warmed up. We know from Greenland and Antarctica ice cores that the Northern and Southern Hemispheres warmed up at different times and at different rates. On top of this there are millennial-scale climate events, when huge amounts of ice broke off from the melting North American ice sheet, flooding the North Atlantic Ocean with freshwater, changing ocean circulation, and cooling the Northern Hemisphere.
One of these events, called Heinrich event 1, occurred about 15,000 years ago, and the other was the Younger Dryas, which occurred about 12,000 years ago. Because of the wonderfully named ‘bipolar climate seesaw’, whenever the Northern Hemisphere cools down heat is exported southwards by the oceans and the Southern Hemisphere warms up. So if you compare an individual ice-core temperature record with reconstructed atmospheric CO2 levels then there will be times when the relationship seems to swap. To really understand the relationship between global temperatures and CO2, Professor Jeremy Shakun of Harvard University and colleagues created a master stack of all the temperature records across the end of the last ice age (see Figure 14). This shows that atmospheric CO2 level leads global temperatures, adding to our confidence that it was contributing to the warming of the Earth as we exited the last great ice age.
14. Global temperatures and carbon dioxide changes for the last 20,000 years.
CO2 is a small part of the atmosphere—it can’t have a large heating effect. This is an attempt to play a classic common-sense argument, but it is completely wrong. First, scientists have repeated experiments in the laboratory and taken measurements in the atmosphere, demonstrating again and again the greenhouse effect of CO2. Second, as for the ‘common-sense’-scale argument that a very small quantity of something can’t have much of an effect, it only takes 0.1 grams of cyanide to kill an adult, which is about 0.0001% of your body weight. Compare this with CO2, which currently makes up 0.04% of the atmosphere and is a strong GHG. Meanwhile, nitrogen makes up 78% of the atmosphere and yet it is highly unreactive.
Every data set has been corrected or tweaked to show global warming. For people who are not regularly involved in science, this seems to be the biggest problem with the whole ‘climate change has happened’ argument. As shown above, all the climate data sets covering the last 150 years require some sort of adjustment. This, though, is part of the scientific process. For example, in 2012, Muller and his Berkeley group published their collated global temperature records and showed an increase of 1.5°C over the past 250 years. This was much higher than other estimates, as the Berkeley group had not corrected all the climate records. Science moves forward incrementally; it gains more and more understanding and insights into the data sets it is using. This constant questioning of all data and their interpretations is the core strength of science: each new correction or adjustment is due to a greater understanding of the data and the climate system, and thus each new study adds to the confidence that we have in the results. This is why the IPCC report refers to the ‘weight of the evidence’, since our confidence in science increases if similar results are obtained from very different sources.
Recent changes in global temperatures are due to changes in the Sun. Both deniers and climate scientists agree that sunspots and volcanic activity do influence climate and global temperatures. The difference between the two camps is that deniers want these natural variations to be the dominant control on climate. There is evidence that the 11-year solar cycle, during which the Sun’s energy output varies by roughly 0.1%, can influence ozone concentrations, temperatures, and winds in the stratosphere. These changes have only a very small effect on surface temperatures. Figure 15 shows that since 1880, the solar radiation increased gradually to a peak in about 1955, and since then it has been decreasing. So over the past 50 years, when global temperatures have increased dramatically, solar output has in fact decreased.
15. Sunspot and global temperatures.
Over the past 150 years, significant changes in climate have been recorded, changes which are markedly different from those of at least the past 2,000 years. These changes include a 1.1°C increase in average global temperatures; sea-level rise of over 24 cm; significant shifts in the seasonality and intensities of precipitation; changing weather patterns; the accelerated melting of the Greenland and Western Antarctic ice sheets; and the significant retreat of Arctic sea ice and nearly all continental glaciers. According to the US National Oceanic and Atmospheric Administration, between 1880 and 2020, the 10 warmest years on record have all occurred within the past 15 years, with 2020 joint warmest year with 2016, followed by 2019, 2015, 2017, 2018, 2014, 2010, 2013, and 2005. The IPCC 2021 report states that the evidence for climate change is unequivocal, and there is very high confidence that this warming is due to human emissions of GHGs. This statement is supported by six main lines of evidence: (1) the rise in GHGs in the atmosphere has been measured and the isotopic composition of the gases shows that the majority of the additional carbon comes from the burning of fossil fuels; (2) laboratory and atmospheric measurements show that these gases absorb heat; (3) significant changes in global temperatures and sea-level rise have been observed over the past century; (4) other significant changes have been observed in the cryosphere, oceans, land, and atmosphere including retreating ice sheets, sea ice, and glaciers, and extreme weather events, all of which can be directly attributed to the impact of climate change; (5) there is clear evidence that natural processes including sunspots and volcanic eruptions cannot explain the warming trend over the past 100 years; and (6) we now have a deeper understanding of the longer term climate changes of the past and the critical role GHGs have played in regulating the climate of our planet.