Getting Sceptical about Global Warming Scepticism

John Cook rebuts the most common sceptic arguments against global warming

A fully referenced and illustrated version of this article is posted on ISIS members’ website, details here, and can be downloaded here

Who’s a sceptic?

A Gallup poll finds only 58 percent of the general public believe human activity is changing global temperatures [1] That is in strong contrast to 97 percent of actively publishingclimate scientists who say humans are a significant contributor [2].Why the great divide between public opinion and scientific experts? Unfortunately, there is no shortage of misinformation and confusion surrounding the climate debate. How does one penetrate the noise to get real scientific evidence? When one scans the many arguments of global warming skeptics [3], a common pattern emerges. Each argument narrowly focuses on a small piece of the puzzle while ignoring the broader picture. This form of cherry picking often leads to erroneous conclusions.

Human CO₂ emissions insignificant compared to natural emissions?

A typical example is the sceptic argument [4] "Human CO2 emissions are small compared to natural emissions". The argument is as follows: “Land and vegetation emit 439 Gt of CO₂ each year while oceans release 332 Gt. In contrast, humans emit only 29 Gt of CO₂ a year. How can humans make an impact on climate when our CO₂ emissions are so tiny compared to natural emissions?”

While these numbers are correct, they don’t tell the full story. This argument fails to disclose that nature both emits and absorbs carbon dioxide. Land and vegetation make up a strong carbon sink, absorbing 450 Gt per year. Similarly, the ocean absorbs around 338 Gt per year. As a result, the net natural contribution is less than zero .

Global carbon cycle

Numbers represent flux of carbon dioxide in gigatonnes [5]

Is it arrogant to presume mere humans could possibly influence the immense, uncontrollable forces of nature? It's not a question of arrogance. It's simply a question of numbers. Humans are emitting29 Gt CO₂ a year [6]; slightly less than half of that is absorbed by the natural carbon sinks, so CO₂in the atmosphere is increasing by 15 Gt per year [7]. Our activities in burning fossil fuels and changing land use have upset the natural balance. The recent rate of increase since the industrial revolution is unprecedented . Furthermore, atmospheric CO₂ is at its highest level in 15 to 20 million years, as documented most recently in ice core data [8].

CO₂ levels (parts per million) over the past 10 000 years

Blue line derived from ice cores obtained at Taylor Dome, Antarctica [9]; green linederived from ice cores obtained at Law Dome, East Antarctica [10]; red line from direct measurements at Mauna Loa, Hawaii [11]

There’s no empirical evidence that CO₂ causes global warming?

How do we know rising CO₂ levels actually cause warming? Sceptics often claim there’s no empirical evidence that humans cause warming [12]. But the evidence is there in the peer reviewed literature, if they had bothered to look. If CO₂ is causing a greenhouse effect, we expect to see less infrared radiation escaping out to space at the wavelengths that CO₂ absorbs energy.

In 1970, NASA launched the IRIS satellite measuring infrared spectra. In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations. Both sets of data were compared to discern any changes in outgoing radiation over the 26 year period [13]. What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as CO₂ and methane (CH₄) absorb energy . Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect". This result has been confirmed using data from later satellites [14, 15].

Change in outgoing radiation spectrum from 1970 to 1996 due to trace gases

'Brightness temperature' indicates equivalent blackbody temperature [13].

When greenhouse gases absorb infrared radiation, the energy heats the atmosphere which in turn re-radiates infrared radiation in all directions. Some makes its way back to the earth's surface. Hence we expect to find more infrared radiation heading downwards. Surface measurements from 1973 to 2008 found an increasing trend of infrared radiation returning to earth [16].A regional studyover the central Alpsfound that downward infrared radiation is increasing due to the enhanced greenhouse effect [17].Taking this a step further, an analysis of high resolution spectral data allowed scientists to quantitatively attribute the increase in downward radiation to each of several greenhouse gases . The downward flux due to CO₂ alone was 26 W m^-2.The results led the authors to conclude that [18] "this experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming."

Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapour is filtered out, showing the contributions of other greenhouse gases [18].

An enhanced greenhouse effect is being observed [19]. The planet is accumulating heat due to increasing greenhouse gases. This is the gorilla in the room for those searching for other causes of global warming – how do you account for all the extra heat trapped by CO₂?

Solar activity responsible for global warming?

The most common sceptic’s approach is to simply ignore the CO₂ effect and focus on other potential causes, the most popular alternative being the sun. After all, the sun supplies almost all of our climate’s energy. This is the major theme of Martin Durkin’s film “The Great Global Warming Swindle” which uses a strong correlation between solar activity and temperature to supposedly prove the sun is the cause of global warming [20]. The graph comparing sun to climate shown in the film stopped mysteriously around 1980. That’s probably because, unfortunately for the theory, any correlation between sun and temperature ends around 1975. At this point, the global climate warmed while solar activity showed a cooling trend. Over the last 35 years while temperatures have been rising, the sun has been going in the opposite direction .

Annual global temperature change (thin light blue) with 11 year moving average of temperature (thick dark blue) [21].Annual total solar irradiance (TSI) (thin light red) with 11 year moving average of TSI (thick dark red). TSI from 1880 to 1978 reconstructed from sunspot numbers [22]. TSI from 1979 to 2009 from satellites [23].

The globe is chilling not warming?

It’s easy to focus on small pieces of the puzzle when those pieces happen to be the regions we inhabit. The cold spell that swept across Eurasia, England and parts of North America through December 2009 and early January 2010 led to many to claim global warming has ended [24]. To gain a proper perspective of this anomalous weather, we need tostep back and look at the broader picture such as a temperature map of all regions north of 30°N .

Map of temperature anomaly for December 2009 at roughly 1000 metres altitude for the region north of 30°N. Areas in orange and red correspond to strong warm anomalies. Areas in blue and purple correspond to cool anomalies (courtesy of National Snow and Ice Data Center (courtesy NOAA/ESRL Physical Sciences Division [25])

While Eurasia and North America are experiencing unusually cold weather, other regions such as Greenland, eastern Siberiaand the Arctic Ocean are experiencing strong warmth. The hottest regions (more than 7 °C above average) are over the Atlantic side of the Arctic, including Baffin Bay and Davis Strait. Unsurprisingly, sea ice extent was below average in this region.

These strong contrasts in temperature are the result of a strongly negative phase of the Arctic Oscillation, caused by opposing patterns of atmospheric pressure between the polar regions and mid-latitudes. Duringa negative phase, pressures are higher than normal over the Arctic and lower than normal in mid-latitudes.In December 2009, the Arctic Oscillation index was the most negative value since at least 1950.

An even broader picture is a global map of temperature anomaly in the last week of December 2009. Here we see that much of the planet is experiencing warmer temperatures than usual, including North-east America, Canada, North Africa, the Mediterranean, and south-west Asia .

Map of global temperature anomalies, December 26 to January 1 [25]

Even surface temperature doesn’t give you the full picture of global warming. The land and atmosphereare a small fraction of the Earth's climate.The vast majority of heat build-up from global warming goes into the oceans. When all the heat accumulating in the oceans, atmosphere and energy required to melt ice sheets and sea ice are tallied up, we find the planet has accumulated 210 x 10²¹J over the past 35 years . This is an average rate of 190 260 GW [27]. Considering a typical nuclear power plant has an output of 1 GW, imagine 190 000 nuclear power plants pouring their energy output directly into our oceans.

Total Earth Heat Content from 1950. The energy to melt ice sheets and sea ice is included in the Land + Atmosphere portion [27]

John Cook studied physics at the University of Queensland, Australia, and majored in solar physics in his post-grad honours year. He runs the Skeptical Science website www.skepticalscience.com, SiS Editors’ top choice for climate change