Climate forcings

The Most Common Skeptical Arguments About Climate Change (from the Grist articles: How to Talk to a Climate Skeptic – see site for images)


Mars and Pluto are warming too

Objection: Global warming is happening on Mars and Pluto as well. Since there are no SUVs on Mars, CO2 can’t be causing global warming.

Answer: Warming on another planet would be an interesting coincidence, but it would not necessarily be driven by the same causes.

The only relevant factor the earth and Mars share is the sun, so if the warming were real and related, that would be the logical place to look. As it happens, the sun is being watched and measured carefully back here on earth, and it is not the primary cause of current climate change.

As for the alleged extraterrestrial warming, there is extremely little evidence of a global climate change on Mars. The only piece I’m aware of is a series of photographs of a single icy region in the southern hemisphere that shows melting over a six year period (about three Martian years).

Here on earth we have direct measurements from all over the globe, widespread glacial retreat, reduction of sea ice, and satellite measurements of the lower troposphere up to the stratosphere. To compare this mountain of data to a few photographs of a single region on another planet strains credulity. And in fact, the relevant scientists believe the observation described above is the result of a regional change caused by Mars’ own orbital cycles, like what happened during the earth’s glacial cycles.

See Global Warming on Mars? from RealClimate for much more detail about this issue.

Turning to the outer reaches of the solar system: in the icy cold and lonely Kuiper Belt was observed a difference in Pluto’s atmospheric thickness, inferred from two occultation observations 14 years apart. But a cursory glance at Pluto’s orbit and atmosphere reveals how ridiculous it is to draw any conclusions about climate, much less climate change, from observations spanning less than even a single season, let alone enough years to even establish the climate’s normal state.

Anyone trying to draw conclusions about what is happening here on earth from all this might as well be from another planet.

Back to Mars for a quick summary:

On Earth, we have poles melting, surface temperature rising, tropospheric temperatures rising, permafrost melting, glaciers worldwide melting, CO2 concentrations increasing, borehole analysis showing warming, sea ice receding, proxy reconstructions showing warming, sea level rising, sea surface temperatures rising, energy imbalance, ice sheets melting, and stratospheric cooling, all of which leads us to believe the earth is undergoing global warming driven by an enhanced greenhouse effect.

One Mars we have one spot melting, which leads us to believe that … one spot is melting.

Forgive me for not being reassured.


It’s the sun, stupid

Objection: The sun is the source of warmth on earth. Any increase in temperature is likely due to changes in solar radiation.

Answer: It’s true that the earth is warmed, for all practical purposes, entirely by solar radiation, so if the temperature is going up or down, the sun is a reasonable place to seek the cause.

Turns out it’s more complicated than one might think to detect and measure changes in the amount or type of sunshine reaching the earth. Detectors on the ground are susceptible to all kinds of interference from the atmosphere — after all, one cloud passing overhead can cause a shiver on an otherwise warm day, but not because the sun itself changed. The best way to detect changes in the output of the sun — versus changes in the radiation reaching the earth’s surface through clouds, smoke, dust, or pollution — is by taking readings from space.

This is a job for satellites. According to PMOD at the World Radiation Center there has been no increase in solar irradiance since at least 1978, when satellite observations began. This means that for the last thirty years, while the temperature has been rising fastest, the sun has not changed.

There has been work done reconstructing the solar irradiance record over the last century, before satellites were available. According to the Max Planck Institute, where this work is being done, there has been no increase in solar irradiance since around 1940. This reconstruction does show an increase in the first part of the 20th century, which coincides with the warming from around 1900 until the 1940s. It’s not enough to explain all the warming from those years, but it is responsible for a large portion. See this chart of observed temperature, modeled temperature, and variations in the major forcings that contributed to 20th century climate.

RealClimate has a couple of detailed discussions on what we can conclude about solar forcing and how science reached those conclusions. Read them here and here.


Climate scientists dodge the subject of water vapor

Objection: Climate scientists never talk about water vapor — the strongest greenhouse gas — because it undermines their CO2 theory.

Answer: Not a single climate model or climate textbook fails to discuss the role water vapor plays in the greenhouse effect. It is the strongest greenhouse gas, contributing 36% to 66% to the overall effect for vapor alone, 66% to 85% when you include clouds. It is however, not considered a climate “forcing,” because the amount of H2O in the air basically varies as a function of temperature.

If you artificially increase the level of H2O in the air, it rains out immediately (in terms of climate response times). Similarly, due to the abundance of ocean on the earth’s surface, if you somehow removed all the water from the air, it would quickly be replaced through evaporation.

This has the interesting consequence that if you could somehow instantly remove all CO2 from the atmosphere, the temperature would begin to drop, causing precipitation to remove H2O from the air, causing even further drops, in a feedback effect that would not end until no liquid water was left, only ice sheets and frozen oceans.

CO2 put into the air by burning fossil fuels, on the other hand, stays in the atmosphere for centuries before natural sinks finish absorbing the excess. This is plenty of time to have substantial and long-lasting effects on the climate system. As the climate warms in response to CO2, humidity rises and increased H2O concentration acts as a significant amplifier of CO2-driven warming, basically doubling or tripling its effect.

An article from RealClimate — “Water vapor: feedback or forcing?” — has a good discussion of this subject.


Water vapor accounts for almost all of the greenhouse effect

Objection: H2O accounts for 95% of the greenhouse effect; CO2 is insignificant.

Answer: According to the scientific literature and climate experts, CO2 contributes anywhere from 9% to 30% to the overall greenhouse effect. The 95% number does not appear to come from any scientific source, though it gets tossed around a lot.

Please see this paper (PDF), the textbook referenced here, and this article at RealClimate.

There is a very important distinction to be made, as you will read if you follow the link to Real Climate, between water vapour’s role in the Earth’s Greenhouse effect and it’s role in climate change. If you were to read through the table of climate forcings in the IPCC report or at NASA’s page about forcings in its GCM, you won’t find water vapour there at all. This is not because climate scientists are trying to hide the role of water vapour, rather it is because H2O in the troposphere is a feedback effect, it is not a forcing agent. Simply put, any artificial perturbation in water vapour concentrations is too short lived to change the climate. Too much in the air will quickly rain out, not enough and the abundant ocean surface will provide the difference via evaporation. But once the air is warmed by other means, H2O concentrations will rise and stay high, thus providing the feedback.


There is no proof that CO2 is causing global warming

Objection: Correlation is not proof of causation. There is no proof that CO2 is the cause of current warming.

Answer: There is no “proof” in science — that is a property of mathematics. In science, what matters is the balance of evidence, and theories that can explain that evidence. Where possible, scientists make predictions and design experiments to confirm, modify, or contradict their theories, and must modify these theories as new information comes in.

In the case of anthropogenic global warming, there is a theory (first conceived over 100 years ago) based on well-established laws of physics. It is consistent with mountains of observation and data, both contemporary and historical. It is supported by sophisticated, refined global climate models that can successfully reproduce the climate’s behavior over the last century.

Given the lack of any extra planet Earths and a few really large time machines, it is simply impossible to do any better than this.

Aside: It is usually interesting to ask just what observations or evidence your skeptic would consider “proof” that global warming is caused by rising CO2 levels. Don’t be surprised if you get no answer!


CO2 doesn’t lead, it lags

Objection: In glacial-interglacial cycles, CO2 concentration lags behind temperature by centuries. Clearly, CO2 does not cause temperatures to rise; temperatures cause CO2 to rise.

Answer: When viewed coarsely, historical CO2 levels and temperature show a tight correlation. However, a closer examination of the CH4, CO2, and temperature fluctuations recorded in the Antarctic ice core records reveals that, yes, temperature moved first.

Nevertheless, it is misleading to say that temperature rose and then, hundreds of years later, CO2 rose. These warming periods lasted for 5,000 to 10,000 years (the cooling periods lasted more like 100,000 years!), so for the majority of that time (90% and more), temperature and CO2 rose together. This remarkably detailed archive of climatological evidence clearly allows for CO2 acting as a cause for rising temperatures, while also revealing it can be an effect of them.

The current understanding of those cycles is that changes in orbital parameters (the Milankovich and other cycles) caused greater amounts of summer sunlight to fall in the northern hemisphere. This is a small forcing, but it caused ice to retreat in the north, which changed the albedo. This change — reducing the amount of white, reflective ice surface — led to further warmth, in a feedback effect. Some number of centuries after that process started, CO2 concentrations in the atmosphere began to rise, which amplified the warming trend even further as an additional feedback mechanism.

(You can go here for a discussion of exactly this question by climate scientists, with greater technical detail and full references to the scientific literature.)

So it is correct that CO2 did not trigger the warmings, but it definitely contributed to them — and according to climate theory and model experiments, greenhouse gas forcing was the dominant factor in the magnitude of the ultimate change.

This raises a warning for the future: we may well see additional natural CO2 come out of the woodwork as whatever process took place repeatedly over the last 650K years begins to play out again. The likely candidates are out-gassing from warming ocean waters, carbon from warming soils, and methane from melting permafrost.


CO2 in the air comes mostly from volcanoes

Objection: One decent-sized volcanic eruption puts more CO2 in the atmosphere than a decade of human emissions. It’s ridiculous to think reducing human CO2 emissions will have any effect.

Answer: Not only is this false, it couldn’t possibly be true given the CO2 record from any of the dozens of sampling stations around the globe. If it were true that individual volcanic eruptions dominated human emissions and were causing the rise in CO2 concentrations, then these CO2 records would be full of spikes — one for each eruption. Instead, such records show a smooth and regular trend.

The fact of the matter is, the sum total of all CO2 out-gassed by active volcanoes amounts to about 1/150th of anthropogenic emissions.


What about mid-century cooling?

Objection: There was global cooling in the ’40s, ’50s, and ’60s, even while human greenhouse-gas emissions were rising. Clearly, temperature is not being driven by CO2.

Answer: None of the advocates of the theory of anthropogenic global warming claim that CO2 is the only factor controlling temperature in the ocean-atmosphere climate system. It is a large and complex system, responsive on many different timescales, subject to numerous forcings. AGW only makes the claim that CO2 is the primary driver of the warming trend seen over the last 100 years. This rise has not been smooth and steady — nor would it be expected to be.

If you look at the temperature record for the 1990s, you’ll notice a sharp drop in ’92, ’93, and ’94. This is the effect of massive amounts of SO2 ejected into the stratosphere by Mount Pinatubo’s eruption. That doesn’t mean CO2 took a holiday and stopped influencing global temperatures; it only means that the CO2 forcing was temporarily overwhelmed by another, opposite forcing.

The situation is similar to the cooling seen in the ’40s and ’50s. During this period, the CO2 warming (a smaller forcing at the time) was temporarily overwhelmed by by other factors, perhaps foremost among them an increase in human particulates and aerosol pollution. Pollution regulations and improved technology saw a decrease in this latter kind of emissions over the ’60s and ’70s, and as the air cleared, the CO2 signal again emerged and took over. Below, courtesy of Global Warming Art, is an image of the current understanding of the factors and their influence for the climate of the past century.

As the graph shows, in addition to aerosol pollution (the sulphate line), volcanic influences were increasingly negative during the period of global cooling, and solar forcing slightly declined. All forcings taken together and run through the model are a very good match for the observations. (Please see the source page for details of what model and what study this image is derived from.)

Rather than confounding the climate consensus, mid-century cooling is actually a good test for the climate models, one they are passing quite convincingly.

Addendum: The opposing effect of cooling from airborne pollutants is often referred to as “Global Dimming”, and Real Climate has a couple of articles on it:

Global Dimming?
Global Dimming II

One emerging concern is that as the pollution causing this effect is gradually cleaned up, we may see even greater greenhouse gas warming.


Geological history does not support CO2’s importance

Objection: Over the last 600 million years, there hasn’t been much correlation between temperatures and CO2 levels. Clearly CO2 is not a climate driver.

Answer: While there are poorly understood ancient climates and controversial climate changes in earth’s long geological history, there are no clear contradictions to greenhouse theory to be found.

What we do have is an unfortunate lack of comprehensive and well-resolved data. There is always the chance that new data will turn up shortcomings in the models and unforeseen new aspects to climate theory. Scientists in the field are working hard to uncover such things — every scientist relishes the thought of uncovering new data that overturns current understanding. But it makes no sense to reject CO2 as a primary driver of climate change today because it looks, through the foggy glasses of time, like CO2 has not always completely controlled climate changes in the past.

The climate system is complicated — even the configuration of the continents has a big effect — so one can not expect complete correlation on all timescales between temperatures and any single factor.


Natural emissions dwarf human emissions

Objection: According to the IPCC, 150 billion tonnes of carbon go into the atmosphere from natural processes every year. This is almost 30 times the amount of carbon humans emit. What difference can we make?

Answer: It’s true that natural fluxes in the carbon cycle are much larger than anthropogenic emissions. But for roughly the last 10,000 years, until the industrial revolution, every gigatonne of carbon going into the atmosphere was balanced by one coming out.

What humans have done is alter one side of this cycle. We put approximately 6 gigatonnes of carbon into the air but, unlike nature, we are not taking any out.

Thankfully, nature is compensating in part for our emissions, because only about half the CO2 we emit stays in the air. Nevertheless, since we began burning fossil fuels in earnest over 150 years ago, the atmospheric concentration that was relatively stable for the previous several thousand years has now risen by over 35%.

So whatever the total amounts going in and out “naturally,” humans have clearly upset the balance and significantly altered an important part of the climate system.


Mauna Loa is a volcano

Objection: CO2 levels are recorded on top of Mauna Loa … a volcano! No wonder the levels are so high.

Answer: Yes, it’s true, Mauna Loa is an active volcano. In fact it’s the biggest volcano on earth! So, should we suppose that Charles Keeling didn’t know that?

Well, no, he did know it. And using subtle scientific indicators like “wind direction,” he was even able to ensure that his readings were not contaminated by any out-gassing when it was occurring. OK, to be fair, it is not really always that simple; out-gassed CO2 can be carried far away on a favorable wind, only to return much later on an ill one. But really, these are clever people, these scientists, and while mistakes are made, they are not usually such simple ones.

A quick look at the actual levels recorded makes it pretty hard to believe there is any volcanic influence. We have a nice, slow, steady trend with a regular up and down seasonal variation. No spikes, no dips. Nothing random, as one would expect from an overwhelming volcanic influence. The record is here among other places.

But, OK, let’s throw out Mauna Loa. There are dozens of other sampling stations scattered all over the globe, including one in the Antarctic, far from cities, SUVs, cement plants, and active volcanoes. It also shows the same rise [PDF], though the southern hemisphere tends to lag a few years behind the northern hemisphere, where the majority of the CO2 is produced. Here are eight others — same results.

Sorry, its all of us Joes, not the volcanoes.


The CO2 rise is natural

Objection: It’s clear from ice cores and other geological history that CO2 fluctuates naturally. It is bogus to assume today’s rise is caused by humans.

Answer: We emit billions of tons of CO2 into the air and, lo and behold, there is more CO2 in the air. Surely it is not so difficult to believe that the CO2 rise is our fault. But if simple common sense is not enough, there is more to the case. (It is worth noting that investigation of this issue by the climate science community is a good indication that they are not taking things for granted or making any assumptions — not even the reasonable ones!)

It is true that CO2 has gone up on its own in the past, most notably during the glacial-interglacial cycles. During this time, CO2 rose and fell by over 100 ppm, ranging between around 180 to 300ppm. But these rises, though they look steep over a 400Kyr timeframe, took 5K to 20Kyrs, depending on the glacial cycle.

By contrast, we have seen an equivalent rise of 100ppm in just 150 years! Check this plot for a dramatic juxtaposition of the slow glacial termination versus the industrial revolution.

There is still more to the case. By analyzing the isotopes of the carbon and oxygen atoms making up atmospheric CO2, in a process similar to carbon dating, scientists can and have detected a human “fingerprint.” What they have found via the isotope signatures can be thought of as “old” carbon, which could only come from fossil fuel deposits, combined with “young” oxygen, as is found in the air all around us. So present day combustion of fossilized hydrocarbon deposits (natural gas, coal, and oil) is definitely the source of the CO2 currently accumulating — just as common sense tells us.

For more of the nitty gritty technicalities straight from the climate scientists, including links to the actual research that established this, visit RealClimate’s article on how we know the CO2 is ours.

Of all the pillars holding up the theory of anthropogenic global warming, this is one of the most unassailable.


The US is a net CO2 sink

Objection: The United States absorbs more CO2 into its land than it emits into the air. The world should be grateful.

Answer: As often the case, at the heart of this talking point is a grain of truth. But it does not serve the purpose for which it’s been enlisted. According to the U.S. Department of Energy land-use changes in the U.S. between 1952 and 1992 have resulted in a net absorption of CO2. But this is only true of natural CO2 — the natural flux of CO2 into and out of forests and peat bogs and soil, as well as carbon that’s been sequestered as lumber and other wood products. These fluxes are actually much larger than anthropogenic emissions, but they go both ways, whereas fossil fuel burning only emits carbon.

This net sink of natural carbon has only been sufficient to offset around 25% of fossil fuel carbon emissions from vehicles, power plants, and the like. In Chapter 7 of this 1996 report, the DoE notes:

For purposes of comparison, this estimated amount of sequestered carbon offset approximately 17 percent of the 1,381 million metric tons of carbon (or 5,068 million metric tons of carbon dioxide) emitted in the United States in 1992 from the burning of fossil fuels.

So at least for 1992, that leaves 83% of fossil-fuel emissions in the atmosphere to spread over the globe or be absorbed into the oceans. In the 2003 report, that number has increased to 83.1%:

The U.S. Environmental Protection Agency (EPA) estimates annual U.S. carbon sequestration in 2003, based on data generated by the U.S. Department of agriculture (USDA), at 828.0 million metric tons carbon dioxide equivalent (MMTCO2e), a decline of approximately 21 percent from the 1,042.1 MMTCO2e sequestered in 1990 (Table 33). Land use, land-use change, and forestry practices offset approximately 16.9 percent of total U.S. anthropogenic carbon dioxide emissions in 1990 and 11.9 percent in 2003. [page 75-76]

With per capita emissions five times greater than the global average, that leaves a lot the world does not have to thank the U.S. for.


Observations show climate sensitivity is not very high

Objection: Taking into account the logarithmic effect of CO2 on temperature, the 35 percent increase we have already seen in CO2 concentrations represents about three-quarters of the total forcing to be expected from a CO2 doubling. Since we have warmed about 0.7 degrees Celsius so far, we should only expect about 0.3 degrees more for a doubling from pre-industrial levels, so about 1 degree total, not 3 degrees as the scientists predict. Clearly the climate model sensitivity to CO2 is much too high.

Answer: Even without addressing the numbers in this argument, there is a fundamental flaw in its reasoning.

We don’t yet know exactly how much the climate will warm from the CO2 already in the air. There is a delay of several decades between forcing and final response. Until an equilibrium temperature is reached, present day observations will not tell us the exact value of the climate’s sensitivity to CO2.

The reason for this is primarily the large heat capacity of the oceans. The enhanced greenhouse effect from higher CO2 levels is indeed trapping energy in the climate system according to expectations, but the enormous quantity of water on earth is absorbing most of the resulting heat. Due to water’s high heat capacity, this absorbed energy shows up as only a modest ocean warming, which in turn dampens the temperature change on land and lowers the global average trend.

This is commonly referred to as the climate system’s thermal inertia. According to model experiments and consistent with data from past climate changes, this inertia results in a lag of several decades between the imposition of a radiative forcing and a final equilibrium temperature.

Now let’s look at a couple of further details. CO2 is not the only factor affecting global temperature. There is a phenomenon called “global dimming” counteracting greenhouse gas warming. Global dimming refers to the blocking of incoming sunlight by particulate pollution in the troposphere and airplane contrails in the stratosphere. It is not a well quantified effect, but it may well be masking a great deal more warming; it is definitely masking some.

This is just one example of why we cannot attribute global temperature trends entirely to CO2 — the same mistaken premise that fuels arguments about the mid-century cooling trend.

I believe it was Richard Lindzen who first made this argument [PDF] about climate sensitivity. The numbers he uses don’t add up. A 35 percent increase in CO2 should correspond to 43 percent of the forcing from two times CO2 (ln(1.35)/ln(2)= 43%), which is not three-fourths.

The original article for this Skeptic Guide entry had an extremely interesting discussion under it, for anyone interested.


If aerosols are blocking the sun, the south should warm faster

Objection: Scientists claim that global warming from greenhouse gases is being countered somewhat by global dimming from aerosol pollution. They even claim that aerosol pollution caused the cooling in the mid-century. But GHGs are evenly mixed around the globe, while aerosols are disproportionately concentrated in the Northern Hemisphere. It follows that warming should be greater in the Southern Hemisphere — but that’s the opposite of what is happening. Clearly climate scientists do not know what is really going on.

Answer: Aerosol cooling does indeed affect the Northern Hemisphere, where most aerosols are produced, more than the southern hemisphere. It is also true that GHGs are well-mixed in the atmosphere — apart from a lag of a few years, southern hemispheric concentrations are rising just the fast as northern.

Where the argument goes wrong is in its assumption that uniform CO2 concentrations imply uniform heating.

It is completely in line with model expectations that CO2-dominated warming disproportionately affects the north. The reasons lie in those complications plaguing the climate system that everyone is so fond of highlighting when it suits the argument and ignoring when it doesn’t. This particular complication is well enough understood to explain what we observe.

GHG forcing has a greater effect over land. The ocean can absorb far more heat without warming nearly as much. It can distribute heat quickly in the upper layers via convection, moving the heat into lower waters rather than warming the upper waters. On land, most extra heat is transferred directly to the air, thus showing up immediately as greater atmospheric warming.

It so happens there’s a disproportionate amount of land in the Northern Hemisphere. The land area of North America, Europe, Asia, the Middle East, most of Africa and the top of South America are all in the north, while the south has Antarctica, Australia, and the rest of South America and Africa.

Another factor is differing ocean dynamics in the north and south. More heat mixes into deeper waters of the Southern Ocean. And sea-ice feedbacks are much greater in the arctic than in the Antarctic.

The arctic also exhibits a polar amplification, which has been observed and modeled. Real Climate has a good article on this.

In short, it’s simply incorrect to say we should see now, or expect to see later, more warming in the Southern Hemisphere. There are mysteries left to solve, but greater warming in the Northern Hemisphere is not one of them.