Attributing Mankind

The hand of Man

There are a few ways we can attribute the CO₂ increase to mankind, primarily to our burning of fossil fuels.[1]

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.[2]

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.[3] 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 1/5^th 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 result

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.[4]

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.[5]

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.[6] 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.

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.[9]

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 1/3^rd 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.[10]

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.[11]

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.[12]

The result is this.[13]

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.