Pointing the finger
The majority of the warming over the last three decades is explained by anthropogenic causes – primarily an enhanced greenhouse effect. However, not everyone is convinced of this. The belief is that mankind is incapable of changing the climate in any significant way.
The hand of Man
There are a few ways we can attribute the CO2 increase to mankind, primarily to our burning of fossil fuels.
The first is decreasing oxygen in the atmosphere. The upper graph shows the famous Keeling curve of increasing CO2. Since the early ‘90s, we have been measuring the oxygen concentration as well. When you burn fossil fuels and biomass, the carbon they contain is affixed to oxygen, creating carbon dioxide. As CO2 increases, oxygen decreases. If volcanoes or the ocean were releasing this CO2, the oxygen concentration would stay the same. Please note these graphs are on different scales. Oxygen far outnumbers CO2 in the atmosphere so this decrease is insignificant compared to the total concentration of oxygen in the atmosphere.
The second piece of evidence represents the smoking gun, shown in the bottom graph. Fossil fuels contain carbon isotopes in different proportions than the carbon found in the atmosphere. As we burn fossil fuels, we can observe the ratio of these isotopes changing, as more and more fossil carbon enters the atmosphere.
Even though many uncertainties remain in climate science, the fact that we are behind the increase in CO2 is not one of them.
“Mankind’s CO2 contribution is small compared to nature.”
Human emissions are only a few percent of what is emitted into the atmosphere naturally. But what nature giveth, it taketh away by way of the carbon cycle. It absorbs as much as it emits. It is a cycle that never ends.
Mankind, on the other hand, giveth and giveth and giveth. The carbon within fossil fuels comes from deep in the ground where it has been accumulating for millions of years. In geological terms, we are effectively releasing it back into the atmosphere all at once, and we are doing virtually nothing to recycle this CO2. In addition, through deforestation we are destroying the natural CO2 sinks of the planet. Burning forests to make way for agriculture or civilization converts previously sequestered carbon into CO2.
What goes up must come down . . . very slowly
CO2 is eventually removed from the atmosphere, but it takes a long time. All things being equal (and they are increasingly not), the ocean and the atmosphere stay in equilibrium. Some CO2 is quickly absorbed by the upper ocean. But the rest takes a long time due to the slow mixing of the deep ocean.
The graph shows the reduction of CO2 after it has been emitted into the atmosphere. It is valid for only relatively modest increases in CO2. If we go beyond that, these numbers don’t apply and emitted CO2 will remain in the atmosphere for even longer periods of time.
Based on this scenario, about half of emitted CO2 is absorbed by the environment in the first 30 years. About 1/5th remains even after 1000 years, so the further into the future we go, the harder it becomes to absorb the last portions of this CO2. What we do today affects the atmosphere for thousands of years!
The ice cores contain a record of the atmosphere going back many hundreds of thousands of years. This graph shows the CO2 concentration of the atmosphere over the past 20,000 years.
The relatively slow build up of CO2 starting about 17,000 years ago is the result of the earth coming out of the last "ice age" - which is more accurately called a "glacial period". Increasing temperatures and retreating ice sheets changes the land, ocean, and plant life. The result is more CO2 in the atmosphere. The grey box on the left shows the range of atmospheric CO2 that has occurred naturally over the entire ice core record going back more than 650,000 years. At no time within this period has CO2 reached a concentration as high as it is today.
Not only is the total concentration unusual, but so is the speed by which humans have added CO2 to the atmosphere. As a result of the last deglaciation, the atmosphere changed over a period of 7000 years. During that time, atmospheric CO2 increased about 100 ppm. Over the past 250 years, human beings have added the same amount, and the result is the huge spike at the present day. The ice core records are detailed enough to tell us that a rapid increase like this has not occurred in at least the last 50,000 years. Currently, 1 out of every 4 CO2 molecules in the atmosphere is the result of human influence. Another way of saying this is that CO2 levels are about 35% higher than pre-industrial levels.
To date, we have added CO2 about 30 times faster than what occurred naturally during the last deglaciation, and this increase is accelerating.
“Temperature change causes CO2 change”
By examining oxygen isotope ratios of the ice, we can also determine the approximate temperature at the time the snow fell. There is some difficulty matching the age of the air bubbles to temperature, because the bubbles are formed only after the snow is compressed into ice, which takes centuries. However, even with these uncertainties, we can still make conclusions.
CO2 and temperature correspond quite well, but the CO2 increase lags the temperature increase, as shown in this graph.
Temperature and CO2 concentration have been offset approximately 800 years so that the rise and fall line up. To skeptics, this lag means that CO2 does not cause the temperature to increase. Instead, it’s the other way around! So there is no need to worry about elevated levels of CO2.
Milankovitch cycles (finally) explained
To understand what is happening, we must return to Milankovitch cycles, which have dominated climatic fluctuations over the past two million years.
These are variations in the Earth’s orbit (E) due to the pull of the other planets, and changes to the tilt (T) and wobble (P) of the Earth’s axis due to the pull of the moon and sun. These cycles act to redistribute energy to different parts of the earth at different times of the year.
When combinations of these cycles interact with the seasons on Earth to enhance and retain winter snowpack, the result is the growth of ice sheets and glaciers. As the ice sheets grow, sea levels drop, exposing more land. The increased ice cover and land surface alters the albedo (reflectivity) of the earth. More energy is reflected back out into space which cools the planet.
Colder oceans and changes to the land and vegetation lead to reductions in greenhouse gases such as CO2, methane, and nitrous oxide. As a result, the greenhouse effect is reduced, which further increases and sustains the cooling.
When the Earth comes out of an ice age, it is largely this process in reverse, although it happens more quickly due to the speed of melting ice. That is, ice sheets disintegrate faster than it takes to build them with falling snow.
CO2 as a feedback
The increase in CO2 and other greenhouse gases is a feedback of the temperature change. The increase is both a result of and cause of warming. Quoting from the earlier paper, “This sequence of events is still in full agreement with the idea that CO2 plays, through its greenhouse effect, a key role in amplifying the initial orbital forcing.”
The skeptics say that this is backpeddling, as if scientists are straining to salvage the CO2/temperature connection. However, the amplifying effect of increasing greenhouse gases was predicted long before the lag was measured.
From this 1990 paper:
“The discovery of significant changes in climate forcing linked with the composition of the atmosphere has led to the idea that changes in the CO2 and methane content have played a significant part in the glacial-interglacial climate changes by amplifying, together with the growth and decay of the Northern Hemisphere ice sheets, the relatively weak orbital forcing and by constituting a link between the Northern and Southern Hemisphere.”
Recipe for an ice age
The changes to the albedo are not enough to explain the swings in temperature that the earth undergoes when transitioning to and from ice ages.
By studying the ice cores, it is possible to quantify the different components that create ice age conditions. During the last glacial maximum, global temperatures were between 4 and 6 ° cooler than today. To get temperatures such as that requires a reduction in radiative forcing around 8 watts per square meter. There are four components that lend themselves to such a change.
The most important is the albedo change caused by the ice sheets and lower sea levels. Not far behind is the reduction in greenhouse gases, among them CO2, methane, and nitrous oxide. Next is the contribution from changing vegetation patterns, also increasing the albedo. Lastly, is the contribution from higher levels of atmospheric dust.
The reduced greenhouse gas concentrations are responsible for about 1/3rd of the total temperature change. Our understanding of the influence of greenhouse gases is actually the best understood of all these factors. The albedo and aerosol changes are harder to quantify.
Pulling the trigger
The precise duration of the CO2 lag during glaciation and declaciation is still being debated, but the fact that CO2 usually trails temperature changes makes intuitive sense. The environment doesn’t decide to create a bunch of CO2 without cause. Some external factor has to act as a trigger.
Skeptics often ask for an example of CO2 increase initiating temperature increase. 55 million years ago, massive amounts of CO2 and methane were released into the atmosphere. Global temperatures increased 5 °C. This is called the Palaeocene-Eocene Thermal Maximum.
The top pane represents a reduction in carbon 13 isotopes in ocean sediment, which implies an increase in atmospheric carbon. The middle pane represents the increase in the oxygen 18 isotope ratio, which is a proxy for temperature. The bottom represents the increasing acidity of ocean waters due to more dissolved CO2.
Current understanding blames intense and sustained volcanic activity that increased the CO2 concentration of the atmosphere. The subsequent increase in temperatures initiated the runaway release of methane from ocean sediments, permafrost, or both, pushing temperatures even higher (see section 12 for more on this tipping point). The increase in greenhouse gas concentrations is roughly equivalent to what will be released in the next 100 years due to human emissions, assuming there is no attempt to reduce them. It took 100,000 years before the atmosphere recovered.
“Water Vapor is 95-98% of the greenhouse effect”
It is often said that water vapor is 95-98% of the greenhouse effect. Water vapor is the most important, and most abundant, greenhouse gas, but it isn’t 95-98% of the greenhouse effect. The source for these figures is unclear. The paper below is often cited as a source, but as the title makes clear, the authors are talking about absorption of incoming solar radiation, and not outgoing infrared radiation which is what we are interested in when we are talking about the greenhouse effect.
Where they might be getting the 98% figure is from the observation that the total greenhouse effect raises the mean temperature of the earth about 33 °. To date, anthropogenic global warming is responsible for about a 0.7 ° temperature increase, which is roughly equivalent to 2% of the natural greenhouse effect.
Wherever they are getting it, it’s not correct. The effects of the various greenhouse gases overlap, so the combined total is less than the sum of the parts. If you take away one gas, another continues absorbing radiation at some of the same frequencies. Because of this, it is somewhat difficult to calculate the individual contribution of each gas.
If we were to instantaneously remove water vapor from the atmosphere, its ability to absorb infrared radiation would drop about 36%. However, if we removed all other greenhouse gases, leaving only water vapor, 66% would be retained. So the contribution of water vapor to the greenhouse effect is between 36% and 66% according to this model.
Performing this same calculation for other greenhouse constituents gives us the figures in the table.
CO2 is between 9 and 26% of the greenhouse effect according to this model.
Despite its low concentration in the atmosphere, CO2 has a significant effect because it well mixed, which means that it is distributed throughout the atmosphere, both by region and by altitude. The presence of water vapor varies greatly from region to region and its influence diminishes as altitudes increase. Adding CO2 has the same effect as throwing more blankets on top of a comforter that doesn’t cover your feet. Even though the comforter does most of the work, the blankets still work to retain heat, and they cover areas that the comforter missed.
Water vapor itself is a feedback of other temperature changes, regardless of their cause. The amount of water in the atmosphere depends on the temperature. The warmer the atmosphere, the more water it can hold and the greater the greenhouse effect. The water vapor feedback is roughly equal to the amount of warming directly attributed to CO2.
Eventually, after listening to all of this evidence and reasoning, a skeptic will demand that you prove it to them. In order to “prove” that humanity is heating up the atmosphere, we’d have to create an experiment with two or more models. This would involve constructing a new planet, identical in every way to the Earth except with no human influence. Then we’d speed up time so we can observe any differences between the two planets.
Obviously, we can’t do that. We are instead running the experiment on our only working model: our home, the earth. So when skeptics demand proof, they are asking the impossible. No amount of evidence will change their opinion if they require an experiment that is impossible to construct.
What we can do
Our confidence is strong based on our understanding of the physics involved, and the match between predictions and observations. But we will not be able to prove the link between anthropogenic greenhouse gases and global temperature until the magnitude of the temperature change grows far beyond what is explainable by natural variability, in which case it will be too late.
Just because we can’t construct a laboratory experiment doesn’t mean we are helpless. We can calculate the effect of changing greenhouse gases using a variety of methods. We can study the climate of the past contained within the ice cores or other proxy data. Unfortunately, accurate information is not always available. We can derive estimates from the various instrumental records, but those are not very long. Rather than studying the broader climate, we can study the effects of specific events. For example, we can study the aftermath of volcanic eruptions such as Mount Pinatubo, for which we have a lot of data.
We can combine all of these observations, each with its own uncertainties, to arrive at a better approximation of climate sensitivity. Climate sensitivity (section 1) is shorthand for the increase in global temperature resulting from a doubling of atmospheric CO2. For example, an increase from 275 parts per million (pre-industrial levels) to 550 parts per million.
The result is this.
The blue lines are the probability derived from the various methods, and the red line is what you get when you take into account all of the calculations and their respective uncertainties. As a result, we are highly confident that the climate sensitivity for doubled CO2 is between 2 and 4 °C, with the most likely result being 3 °. This is about half of what Arrhenius calculated in the 1890s (section 1) and about 4 times the famous Rasool and Schneider paper from 1971 (section 3). It is right within the range of calculations from the 1979 Charney Report (section 1), and every other consensus report since then.
Fingerprints of anthropogenic influence
How do we know what scientists are telling us is trust worthy? The foundation of our understanding of the greenhouse effect is the measurement of the infra-red absorbing properties of various greenhouse gases. We’ve been doing this for 150 years. We can compare what we know about these gases with what we’ve observed.
In 1970, the Nimbus 4 spacecraft measured the outgoing infrared spectra of the Earth. Starting in 1996 and continuing into 1997, the Japanese ADEOS satellite made the same measurements. If the increasing greenhouse gas concentration has changed the amount of outgoing infrared radiation, these instruments should have measured it.
The top graph contains three lines showing a change in brightness temperature at different frequencies. The top line shows the difference between the 1970 and 1997 spectra for the central Pacific. The middle line shows the simulated difference, based on known changes in greenhouse gases. The measured and simulated differences are virtually identical given the precision of the instruments. The outgoing radiation changed just as we expected it to change. The bottom line shows observed changes over a larger portion of the globe, between 60 °N and S.
Assuming an enhanced greenhouse effect, the nights should be warming faster than days. The greenhouse effect caused by CO2 and other well mixed greenhouse gases operates 24 hours a day. The direct warming influence of the Sun would have the largest impact on daytime temperatures. If the warming was caused by an enhanced greenhouse effect, we would see a decrease in cold nights greater than the decrease in cold days, and we would see an increase in warm nights greater than an increase in warm days. That is exactly what the observations tell us is happening.
Another fingerprint is a cooling stratosphere. More CO2 in the stratosphere has the opposite effect that it has in the troposphere. Another reason for stratospheric cooling is ozone destruction caused by CFCs, which plays a dominant role in the lower stratosphere.
All of this is directly borne out by satellite observations. The cooling caused by increased CO2 levels most strongly occurs at altitudes between 40 and 50 km, which roughly corresponds to measurements taken by two satellite instruments (SSU47x and SSU27) summarized in the next graphs.
Increasing tropospheric temperature and decreasing stratospheric temperature allows us to make another prediction based on our understanding of atmospheric processes. As the difference between the troposphere and stratosphere temperatures increases, we should see an increase in westerly winds at high latitudes. The result should be warmer temperatures over northern continents in the winter, as relatively warm ocean air is blown over relatively cold continents.
The top map shows the temperature anomalies for December through February of the ten year period between 1998 and 2007. The bottom map shows the anomalies over the entire year. You can see that the strongest anomalies occur over northern continents, and the warming is greater during these months than over the entire year.
 (Forster, et al., 2007) Online here. Figure 2.3
 (IPCC, 2001) Online here
 (Hansen, Sato, Kharecha, Russel, Lea, & Siddall, 2007) Online here
 (Jansen, et al., 2007) Online here. Figure 6.4a
 (Caillon, Severinghaus, & Jouzel, 2003) Online here
 (Jansen, et al., 2007) Online here. FAQ 6.1, Figure 1
 (Caillon, Severinghaus, & Jouzel, 2003) Online here
 (Lorious, Jouzel, Raynaud, Hansen, & Le Treut, 1990)
 (Forster, et al., 2007) Online here. Adapted from figure 6.5.
 Ibid. Figure 6.2.
 (Freidenreich & Ramaswamy, 1993) Abstract here
 (Schmidt, 2005) Online here
 (Annan & J.C., 2006) Online here
 (Trenberth, et al., 2007) Online here. Based on FAQ 3.3 figure 1
 (Baldwin, et al., 2007) Online here. Cropped Figure 5-5
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