In the greenhouse
First things first: are we absolutely sure that the Earth is warming? Are we certain that it’s not just due to sunspots or solar flares, or a wobble in the Earth’s orbit around the sun. What about the Milankovitch cycle? And what if all these variables that might warm up the planet just happen to be taking place at the same time?
It certainly seems as if the Earth is getting warmer. Every year since 2015 the meteorologists have been telling us that the year just finished was the hottest ever recorded, and that the present year is on track to be hotter still.
The World Meteorological Organisation’s report on the state of the global climate in 2016 is clear. The year 2016 was the warmest on record. The five- and ten-year mean temperatures also reached their highest values on record, with 2012-2016 and 2007-2016 respectively being 0.65°C and 0.57°C above the 1961-1990 baseline average value. Each of the last 16 years since 2001 has been at least 0.4°C above the 1961-1990 average—a mark which prior to 2001 had only been achieved once– in 1998 (a famous El Nino year). Global temperatures continue to be consistent with a warming trend of 0.1°C to 0.2°C per decade .
Warming extended almost worldwide in 2016—with temperatures above average levels over most of the Earth’s land area. But there were some significant differences. Most mid-and higher latitude areas of the northern hemisphere were at least 1°C above the baseline value. Annual temperatures at least 3°C above average occurred in several high-latitude locations, particularly along the Russian Federation coast, in Alaska, and far north Canada. In the high Arctic, temperatures were significantly above average values—with Svalbard airport in Norway (right up against the Arctic circle) recording average temperatures a huge 6.5°C above the baseline value .
The strong warming trend has continued through 2017—which was the second warmest year on record after 2016.
The figure below is from NASA’s Goddard Institute for Space Studies (GISS) and shows the trend in land surface air temperatures and sea surface water temperatures since 1950 . Although there are periods when warming seems to slow, the trend is inexorably upwards. More alarmingly, the rate of increase in the warming trend since 2010 has increased substantially. You can see a definite uptick in the curves after about 2010.
The global sea surface temperature trend so far for the 21st century is estimate at 0.16°C per decade—a much higher figure than the longer term 1950-2016 trend of 0.10°C per decade .
The rate of energy increase in the climate system is the most fundamental metric that defines the rate of global climate change. More than 90% of the Earth’s energy imbalance goes into heating the oceans. So tracking ocean temperatures and calculating the changes in ocean heat content (OHC) is where you look first if you want to see what’s going on.
Looking at the figure above, you might say that temperature increases of about 1.3°C for land surface air temperatures and 0.6°C for sea surface water temperatures over the last 65 years are no big deal?
You’d be wrong.
To understand what’s happening we need to look at the oceans. The temperature of any material is an indication of its energy content, and the energy absorbed and held in the oceans is orders of magnitude greater than the energy content of the atmosphere.
Water is about a thousand times heavier than air, and it takes almost four times as much energy to raise the temperature of a kilogram of water by 1°C than it does to raise the temperature of a kilogram of air by the same amount.
How much water is in the oceans? About 1.35 billion billion tons.
So raising the temperature of this mass of ocean water by just 0.6°C requires a truly phenomenal amount of energy. Only when you consider the increasing heat content of the oceans, do you start to get an idea of the massive amount of energy that is being absorbed by the planet.
The figure below from the US agency NOAA shows global ocean heat content data since 1957. For the last 50 years, the long-term trend has been strongly positive .
So what’s causing this seemingly inexorable rise in global temperatures?
The Climate Science Special Report, published in 2017 by the US Global Change Research program, is very clear on this question. It states:
This assessment concludes, based on extensive evidence, that it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence .
The boldface text is in the original text–which suggests that the scientists who authored the report wanted to make it absolutely clear that they believe that human activities, especially the emissions of greenhouse gases, are causing global warming.
The discovery of global warming is a fascinating tale of scientific endeavor, insight, perseverance, error, and forgetfulness that starts way back in the early 19th century. In fact, the aim at that time among the scientific community was to understand more about the ice ages. Could another one occur again?
Beginning with work by Joseph Fourier in the 1820s, scientists had speculated that gases in the atmosphere might trap the heat received from the sun. An amateur English scientist, David Tyndall, working out of his makeshift laboratory identified several gases that absorbed radiant heat. He discovered that the most important of these were water vapor and carbon dioxide . Fast forward a century and a half and scientists with immeasurably more accurate and sensitive equipment, and an new understanding of the chemical isotopes of carbon, were gradually unravelling the complexity of the carbon cycle and the role of atmospheric carbon dioxide (CO2) in absorbing infrared thermal radiation from the planet, and thus keeping the planet a comfortable 33°C warmer than it would be if the atmosphere was completely transparent to this outgoing thermal radiation. This is the so-called greenhouse effect.
The figure below shows how it all works .
It looks complicated—and it is.
But the numbers are not as important as the concept. On the left-hand side, in yellow, is the incoming solar energy striking the top of the atmosphere (TOA) with a power level of 340 Watts per square meter. This level of solar energy is pretty much constant. A bit less than half of this energy reaches the surface of the Earth (where it shows ‘solar absorbed surface’). Warming the surface. The Earth, since it is a warm body, emits its own thermal radiation (on the right in orange) but at a much longer wavelength than the incoming radiation from the sun. A fraction of this longwave radiation is absorbed by the greenhouse gases (mainly water vapor and carbon dioxide) — which warms the atmosphere and which, in turn, radiates a part of this energy back to the surface. This radiative energy is shown on the right as ‘thermal down surface’. The net result of these energy flows is tucked away in the bottom left corner. There is an imbalance: more energy is coming into the surface of the Earth than is leaving.
Result? The Earth is warming up. It has to—there is more energy coming in than going out.
And let’s not confuse a warming climate with only warmer weather. It still gets cold in the winter in the northern hemisphere. There’s plenty of snow.
At the same time in the southern hemisphere summer, it is hot. Very hot. In January 2018, South Africa was in the grip of an intense drought; Cape Town is running out of water; New Zealand was sweltering under a heat wave, and in Australia temperatures of over 45° C killed thousands of flying foxes and volunteers were hosing down heat-stressed koalas.
The greenhouse gases
So what are the gases in this greenhouse? Although water vapor is a significant absorber of infrared radiation and is therefore an important greenhouse gas, research has shown that it is carbon dioxide which has a much stronger influence on the global energy budget. It’s been called the control knob: turn it up and the Earth warms; dial it down and the planet cools .
In 1953, a post-doctoral student called Charles Keeling began working at Caltech in California on the chemistry of carbonates in surface waters, and their equilibria with limestone and CO2 in the air. To investigate the chemistry of these compounds and their interactions with CO2, Keeling had to measure CO2 extracted from the air as well as in acidified samples of water. What he found was intriguing. The air contained more CO2 at night than during the day—a consistent diurnal variation. In the afternoon, concentrations were relatively stable at about 310 parts per million (ppm).
The early results of this research led to a larger research program intended to measure concentrations of atmospheric CO2 more widely around the globe. Four monitoring points around the world were proposed but only one, at Mauna Loa in Hawaii, was able to measure CO2 concentrations almost without interruption. In March 1958, CO2 was measured at 313 ppm. More surprising still as the daily measurements were carefully recorded over the course of the year, was a marked seasonal variation in CO2 concentrations as the gas was absorbed for plant growth during the spring and summer months, and returned to the atmosphere during the winter that followed. As Keeling continued to monitor CO2 concentrations and report his results, the now famous saw-tooth graph of increasing atmospheric CO2 concentrations began to take shape . Its present version is shown in the figure below .
The Keeling curve
What is immediately obvious is that the concentrations of CO2 in the atmosphere is increasing, and has been increasing continuously since measurements began in 1958. Although it is a little hard to detect, the rate of increase is also higher now than it was a few years ago. So there is no sign yet that the amount of carbon dioxide in the atmosphere has started to level off. It’s still increasing—just the way it has done, year after year, since the start of the industrial era in the mid 18th Century.So where is all this carbon dioxide coming from?
It’s coming from the burning of fossil fuels: coal, oil, and natural gas. And there’s proof.
An isotope of carbon, called carbon-14 is created by cosmic rays in the upper atmosphere. More of the isotope was created by nuclear weapons tests in the 1950s. The isotope decays very slowly—over thousands of years. However, the carbon in coal and oil is so old that it completely lacks the radioactive isotope. Therefore the emissions of carbon dioxide from burning fossil fuels adds only plain old carbon to the atmosphere. In 1955, the chemist Hans Seuss reported an analysis of wood from trees grown over the last century, reporting that the newer the wood, the greater the ratio of plain carbon to carbon-14—meaning that the amount of plain carbon in the atmosphere was increasing. The only plausible source of this carbon was the burning of fossil fuels: coal and oil .
Although carbon dioxide is the principal actor, he is by no means alone on the stage. There are two other actors of note : methane and nitrous oxide. And three with minor roles: HFCs, PFCs, and sulphur hexafluoride (SF6) . Carbon dioxide, methane, and nitrous oxide together account for more than 80 percent of the warming effect of the greenhouse gases—so on this webiste we are going to focus on these three gases.
Like carbon dioxide, levels of both methane and nitrous oxide are increasing in the atmosphere. Although the concentrations are very low (measured in parts per billion), these are powerful greenhouse gases. The Global Warming Potential (GWP) of methane and nitrous oxide is respectively 25 and 298 times the value of carbon dioxide .
Global atmospheric levels of methane (left) and nitrous oxide (right) 
So the Earth is warming a couple of degrees Centigrade? Big deal… Why worry?
Let’s take a look at heat waves, wildfires, droughts, floods, storms, food insecurity, melting ice sheets, disappearing glaciers, sea level rise, migrating species, and climate change induced conflict. These impacts and more are on the other pages.
For a deeper dive:
 WMO statement on the state of the global climate in 2016. World Meteorological Oganisation WMO-No.1189. Geneva, Switzerland. 2017
 The values are from the WMO report: ibid.
 See the 2017 report from the American meteorological Society: State of the Climate in 2016.
 See the Climate Science Special Report. USGCRP. 2017. Climate Science Special Report: Fourth National Climate Assessment, Volume I. US Global change research program, Washington DC USA. 470 pp. Available online at https://science2017.globalchange.gov
 The schematic is from the Climate science special report (cited above)—which has a more details about the provenance of the numbers shown in the figure.
 Lacis, A.A, Hansen J.E., Russell G.L., Oinas V, et al. The role of long-lived greenhouse gases as principal LW control knob that governs the global surface temperature for past and future climate change. Tellus B 2013, 65.
 See The history of the Keeling curve, by Bob Monroe, accessed on 22 September 2107 at https://scripps.ucsd.edu/programs/keelingcurve/2013/04/03/the-history-of-the-keeling-curve/
 From https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/mlo_full_record.png. Accessed on 23 September 2017
 From the American Institute of Physics article: The discovery of global warming: The carbon dioxide greenhouse effect. Cited above.
 HFCs are hydrofluorocarbons and PFCs are perfluorocarbons. Sulfur hexafluoride is a potent greenhouse gas, produced by the chemical industry and used as an electrical insulator in power distribution equipment. Chlorofluorocarbons together with other halogenated gases contribute about 12 % to radiative forcing by long-lived greenhouse gases. Some hydrochlorofluorocarbons and HFCs are increasing at a relative rapid rate, although they are still low in abundance. See World Meteorological Organization (WMO) Greenhouse gas bulletin No 12. 24 October 2016.
 For readers who really want to get into the weeds on the Global Warming Potential of the other greenhouse gases, there are several excellent reports available on the internet. See for instance : Inventory of US Greenhouse gas emissions and sinks 1990-2015, US Environmental Protection Agency (EPA 430-P-17-001) 2017, for a good review of the science.
 US Environmental Protection Agency: Inventory of US greenhouse gas emissions and sinks 1990-2015. EPA 430-P-17-001. 2017. The global warming potential of methane is 25 over a 100-year period. Since methane only stays in the atmosphere for about 12 years, employing this value is disputable. Many scientists now prefer to calculate methane’s warming potential over a 20-year period–in which case its GWP is closer to 86. This adjustment has important implications when assessing the global warming effect of fugitive methane emissions from natural gas production sites and their gathering systems of pipelines.
 The graphs are from : WMO Statement on the State of the Global Climate in 2016, World Meteorological Oragnization, WMO-No.1189. 2017