Global Warming: The Evidence

Global warming remains a highly contentious and politicised issue, and is one of many unfortunate examples in which the scientific evidence and consensus is subject to denial by large portions of the population.

What is clear is that global warming is not a contentious issue within the scientific community. The Intergovernmental Panel on Climate Change has stated:

Scientific evidence for warming of the climate is unequivocal.

Therefore, it’s unsurprising that a 2013 study found that around 97% of climate scientists agree that global warming is occurring, a figure derived from both evaluations of the abstracts of 4,013 peer-reviewed scientific papers as well as 1,200 authors’ own ratings of their peer-reviewed scientific papers in terms of the position they took on anthropogenic global warming. Indeed, the latter approach found that “97.2% endorsed the consensus” that the theory of anthropogenic global warming is in line with the evidence, while the former found that 97.1% endorsed the consensus.

Some climate change deniers have attempted to undermine this consensus, claiming that only 52% of meteorologists believe that global warming is mostly caused by human activity. The information that they often omit is that the survey which found this only included the opinions of 25% of people in the American Meteorological Society, many of whom are not experts in the field of climate science. Indeed, the survey in question actually found that among the meteorologists whose primary field of expertise is climate science, “78% of climate experts [in the survey] actively publishing on climate change” believe that humans are the primary cause of global warming. It’s also important to stress that the 97.2% figure comes from a survey of thousands of peer-reviewed scientific studies in the field of climate science from around the world. This survey, by contrast, collects the opinions of certain meteorologists in a certain meteorological society in the United States, and even then shows a clear majority (78% of climate experts) who believe that humans are the primary cause of global warming. In addition, the 97% figure was found in a study conducted three years prior to the aforementioned one, finding:

97–98% of the climate researchers most actively publishing in the field surveyed here support the tenets of ACC outlined by the Intergovernmental Panel on Climate Change.

This demonstrates that there has been nothing in the years between 2010-2013 which has undermined the credibility of the theory of anthropogenic global warming.

The Evidence

The first line of evidence which may demonstrate that humans are causing global warming is the correlation between the CO2 levels and global temperatures – as one has risen, so has the other. It is widely known that humans are contributing largely to the rise in CO2 levels through combustion and deforestation. But, correlation does not equal causation. To find out whether something is causing something, we need a mechanism, and the mechanism by which CO2 traps heat was discovered as early as the 19th Century by Joseph Fourier, and expanded upon by John Tyndall and others: the greenhouse effect.

Thus, we have established that humans are contributing to a rise in carbon dioxide which can and does plausibly raise temperature levels. Nevertheless, the extent to which humans are causing temperature levels could still be limited by other factors. However, there is ample evidence which suggests that other factors are not playing a major role in global warming. The first reason to doubt the major role of other factors is that ice cores in Greenland show that non-artificial climate change has, in the past, taken place over thousands and millions of years, whereas global warming that is occurring today is taking place in tens of years.

One specific factor which many global warming skeptics and deniers state may be driving the warming trend is the sun. Nevertheless, since 1975, the sun has shown a slight cooling trend, which continued through the 2000s and resulted in an extremely deep solar minimum from 2007-2009. Taking into account pre-1975 and post-1975 data, Benestad and Schmidt concluded, in their 2009 study:

[T]he most likely contribution from solar forcing a global warming is 7 ± 1% for the 20th century and is negligible for warming since 1980.

Additionally, cosmic rays have been hypothesized to be contributing to global warming, but numerous papers have discredited this hypothesis. A September 2013 paper, for instance, concluded:

…there is little empirical evidence that links GCR [Galactic Cosmic Rays] to the recent global warming.

The predicted effects of global warming are also being observed. For example, according to the National Oceanic and Atmospheric Administration in the United States, global sea levels rose by 1.7mm per year in the last century, corresponding to an increase of 17cm overall. Overall glacier and ice sheet volume is also declining rapidly. Specifically, Arctic sea ice has been steadily declining by 3 to 4% per decade due to climate change. While, in the Antarctic, the surface has been warming at a rate of 0.1C per decade, meaning that the Antarctic Ice Sheets, overall, lost approximately 1350 Gt between 1992 and 2011 at an average rate of 70 Gt per year. Indeed, Antarctica is losing ice mass at an accelerating rate, despite the claim often made by deniers that Antarctica is experiencing in ‘increase in ice extent’, because what they’re actually referring to is the temporary winter sea ice extent, the increase in which was explained in this 2009 study, which found:

The autumn increase in the Ross Sea sector is primarily a result of stronger cyclonic atmospheric flow over the Amundsen Sea.

Essentially, the hole in the ozone layer above the South Pole has caused stratospheric cooling, creating cyclonic winds which push sea ice around, which in turn creates polynyas, areas of open water which contribute to increased sea ice production.

The global warming ‘pause’ is largely a myth

A typical claim from a global warming denier that one will hear today is that Earth has not warmed since 1998. This is a typical anti-scientific claim, as it involves cherry-picking specific periods of time to prove a point. What’s more, using data from 1998 onwards is particularly egregious because the period between 1997-98 included an El Nino event, which caused temperatures to soar in 1998: it was the warmest year on record at the time. After such an unusually warm year in which temperatures were far above the mean, regression to the mean invariably occurred, causing it to superficially appear as if there was a ‘pause’ or ‘cooling’.

Despite this regression to the mean, recent data have even shown that 2010 and 2005 were warmer than 1998, and that 9 out of the 10 warmest years on record have occurred in the 2000s. Therefore, the decade of 2000-2010 was the warmest full decade on record, and 2011-2020 is, at the moment, looking as if it will be even warmer. I also say ‘recent data’ because the UK Met Office’s HadCRUT3 dataset was updated in 2012 to HadCRUT4, which took into account more measurements around the Arctic area, resulting in higher temperatures for 2010 and 2005 relative to 1998, a finding corroborated by NASA’s GISS dataset. Therefore, warming hasn’t even paused.

Nevertheless, it is true that the rate of warming has slowed. The most likely explanation for this is that more than 90% of the Earth’s heat goes into the oceans. Indeed, when we factor in ocean temperatures, global average temperatures are actually accelerating, with this 2013 study stating:

In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend.

The available empirical evidence therefore points to the likely possibility that the strengthening of ocean heat uptake efficiency is a significant cause of the slowing down of the warming of the Earth’s surface. As Masahiro Watanabe and his colleagues put it in this study, the so-called ‘pause’ likely has

…an internal origin… associated with active heat uptake by the oceans.

While ocean heat content seems to be a significant factor in the so-called ‘pause’, as confirmed even more recently by another study looking specifically at the Atlantic Ocean, it has also been suggested in one study that if we had today’s modelling technology in the 1990s, the slowdown in the rate of surface temperature warming could have been predicted, implying that the slowdown may have partly been due to natural variability.

In light of the evidence, the UK Met Office makes three conclusions:

First, periods of slowing down and pauses in surface warming are not unusual in the instrumental temperature record. Second, climate model simulations suggest that we can expect such a period of a decade or more to occur at least twice per century, due to internal variability alone. Third, recent research suggests that ocean heat re-arrangements, with a contribution from changes in top of the atmosphere radiation, could be important for explaining the recent pause in global surface warming.

Conclusions

Across the world, extreme weather conditions are becoming more prevalent. The National Oceanic and Atmospheric Administration has detailed records of this for the United States. Meanwhile, the UK witnessed its wettest winter on record in 2013, in line with the prediction of global warming that increased convection and evaporation will result in more instances of extreme rainfall events. And, in Australia, 2013 was the warmest year on record.

Ideological concerns that action on global warming may undermine a particular economic or political system never justifies denial of the evidence in support of the theory of AGW. In fact, studies are beginning to show that action on global warming doesn’t have to slow down economic growth at all. In essence, there are plenty of potential solutions from across the political spectrum, which should each be put forward and evaluated on their merits, but denial of the problem itself should never be resorted to.