Global Warming: a brief background

I'm writing this for an audience that isn't short on science or Google chops. I'll try to supply links where useful.

We're here to discuss global warming, and several levels of skepticism expressed about global warming.

• What is global warming (GW), and does it really exist?
• Is global warming anthropogenic (AGW)? That is to say - are human activities principally responsible for global warming?
• Is global warming catastrophic (CAGW)? Are the changes severe, irreversible and creating misery for humans and other life on the planet?

A basic issue of terminology - the problem we'll be discussing is one of heat, and heat is measured by temperature. However, there is not a one-to-one correspondence between the two ideas.

It's a gas!

Lets start with the basic physics of the gases that make up Earth's atmosphere. Our atmosphere is mainly composed of nitrogen, oxygen, water, argon, and carbon dioxide in order of abundance. We can see through the atmosphere quite well, in other words, it is transparent to photons. If the photons streaming down at us from the Sun were caught by the molecules and atoms of the atmosphere, we would not be able to see through it.

(Of course, the atmosphere is not perfectly transparent, which is why we see light from the whole sky, not just directly from the sun.)

The Sun supplies the vast majority of heat to the surface of the planet. Heat due to radioactivity in the interior is a negligible source, as is the heat from chemical burning such as forest fires and anthropogenic sources. It would be a really dumb skeptical argument to say "heat from burning in power plants and car engines is too little to cause global warming, therefore GW isn't real." That argument misses the big idea, that human burning activities release carbon dioxide (principally) into the air.

Light travels into the Earth's atmosphere and through it. The light strikes land or water and is absorbed. These are high energy photons of visible light. The water and land release this absorbed energy as longer wavelength infrared radiation.

If the atmosphere was as transparent to infrared as it was to visible light, this would just travel back up through the atmosphere and out to space. However, it is not.

Some molecules trap the infrared as it is radiated by the land and sea. Which molecules? We've known since the 19th century that molecules with combinations of different atoms will absorb infrared energy. Molecules that are made of pairs of the same atoms (such as N2, O2, and H2) do not absorb infrared. Gases that are single atoms, such as argon, xenon, and neon, also don't absorb infrared.

Water (H2O), carbon dioxide (CO2), methane (CH4), and other more complicated molecules do absorb infrared. In general, the more different the atoms in the molecule are, the more there are electrically positive and negative 'ends' to the molecule, the more angles are involved, the more that molecule can absorb infrared.

These differences in properties explain why water is a more potent absorber of IR that CO2. H2O has a bond angle that makes it look like a boomerang, with pronounced negative and positive charge distribution. CO2, on the other hand, has a bond angle of 180 degrees - all the atoms are on one line. As a result, CO2 has fewer ways to absorb IR radiation than H2O.

All of these molecules that absorb IR have been called greenhouse gases (GHGs) because they help warm the planet. If there were no water or CO2 in the atmosphere, we could have 10 times as much nitrogen or oxygen, it wouldn't make the planet warmer. If the planet had a much thinner atmosphere, without the nitrogen or oxygen, it would still have a similar surface temperature as it does now.

Some skeptical arguments start with the idea that CO2 is a 'trace gas', and therefore must be a small contributor to the overall heating process. This is simply false, based on the science of how molecules absorb IR. The small contributors are nitrogen and oxygen, even though they make up the majority of the atmosphere.

As an aside, some folks with an interest in Mars colonization have suggested that the Martian atmosphere be pumped up with relatively small amounts of very powerful greenhouse gases. Only enough would be necessary to raise the temperature at the poles to cause the CO2 ice there to sublimate into the atmosphere and contribute its own warming power. The resulting Martian atmosphere would still be thin and unbreathable, but warm.

Stepping back from Terraforming Mars, we must consider how we are Terraforming Terra - that is, changing our planet by changing the balance of GHGs in the atmosphere.

Even the scientists who first studied the IR absorption of gases speculated that mankind's contribution of CO2 to the atmosphere could warm the planet. One such was Svante Arrhenius. I say Arrenhius was speculating because while he knew the underlying mechanism, he had no way of measuring the effects. As it happened, he also thought that the overall balance of warming the Earth would be beneficial. This idea still recurs in skeptical thinking - "I don't believe AGW is real, but if it was real, it would be a good thing."

But is it Anthropogenic?

Starting in the 1950s, careful measurements have been made of the gas composition of the atmosphere. In general, the atmosphere of the Earth is well mixed. There are places such as the poles where that isn't true in very specific ways, such as the ozone hole over the South Pole. The measurements are made at an atmospheric lab at the top of Mauna Loa, Hawaii so that there are no nearby factories or cities to make interpreting the results difficult.

This series of measurements shows a steady rise in CO2 throughout the last half of the 20th century. In fact, CO2 has risen 38% since 1750, most of that in the last 50 years. (EPA source) All very well, a skeptic might say, but is the source of this CO2 anthropogenic? Yes. 

CO2 from anthropogenic burning of fossil fuels is releasing into the atmosphere carbon that has been sequestered under the earth for millions of years. This carbon was originally plant material, and plants prefer to use carbon-12 (instead of the heavier C-13 or the radioactive C-14). The atmosphere had a particular signature of isotopes C-12 to C-13 ratio, and that ratio has been changing as CO2 has entered the atmosphere. This new CO2 also has little C-14, because almost all of this has decayed away over time in fossil fuels. 

This is the signature of old, fossil carbon entering the atmosphere, and that is the key to attributing the rising CO2 to mankind. The case is even clearer with all of the other GHGs, some of which are unnatural and only come from industrial sources. In addition, there are no obvious natural sources that can account for the trend - no steady rise in the number of volcanic eruptions, for example.

Why focus on CO2?

At this point, I'll say that CO2 is the most talked about GHG, even though it is not the most potent or the most plentiful. Water is the most plentiful GHG and is more powerful than CO2. Other, more exotic gases are much more powerful.

So why focus on CO2? The best reason is what is known as residence time. CO2 stays in the atmosphere a long time. On average, a CO2 molecule will float around for thousands of years before being broken down into something else or used by a plant. In comparison, water molecules only stay in the atmosphere for an average of 11 days! Methane is more powerful, but has a residence time of 15 years. (Methane's short residence time is the reason that most concern over the release of methane from the thawing permafrost or from under the Arctic Ocean is overblown.)

So CO2 is not a very powerful GHG, but it is the 2nd most plentiful GHG after water and has a much longer residence time. That makes it important. Water is important to the background level of global warmth, but not to changes in that level. The overall humidity of the atmosphere is not changing with a clear, dangerous trend.

Heat vs. Temperature

Now that we've identified anthropogenic CO2 entering the atmosphere, our understanding of the physics of CO2 would lead us to expect an increase in heat trapped by this gas. This heat can go many places in the total system of the surface of the planet, and only some of those places will change the temperature of the planet.

An easy example is the melting of the Arctic ice cap. The temperature of a mixture of ice and water will stay at 32F, even while that mixture is absorbing a tremendous amount of heat. The heat is being used to melt the ice. In general the heat capacity of the world's oceans is very large, and water can absorb much more heat than the air above it for a given change in temperature.

This brings us to the difficult point of measuring global changes in absorbed heat by global changes in temperature. Global warming is a problem of more heat, but the temperature of land, air, or sea can poorly reflect that. Indeed, the temperature of the land, sea, and air can all vary from each other significantly.

Other Forcings

There are many drivers of global heat and therefore temperature, besides GHGs. The Sun itself varies in power, and the distance of the Earth, and wobbles in the Earth's orbit also have strong effects. These wobbles, collectively known today as Milankovitch cycles, are responsible for the current pattern of ice ages (in concert with the position of the continents due to plate tectonics). These are long term sources of variation. Shorter term sources include volcanoes, which produce dust and aerosols that cool the planet by reflecting more sunlight into space. 

Even jet contrails can have an effect. Contrails act like other clouds. Clouds during the day reflect light away and cool the planet. Clouds at night reflect IR back down and warm the planet.

In the geek-speak of climatology, all of these different effects are called forcings and they are measured in the currency of Watts per square meter per year, in order to keep them comparable.

Absent the effect of AGW, the consensus is that the Earth's climate would gradually be getting colder, as we head into another Ice Age. We have been in an interglacial period for the last 10,000 years or so, but this is ending.

Given that background, we would expect that the temperature trend would be downward, with some natural volatility representing the interplay of all the different forcings. And it was indeed downward, until the onset of the Industrial Revolution. Since that time, global temperatures have trended upwards, again with some natural volatility.

Much is made in the skeptical debate about the temperature trend, whether it has reached a record for the last x-thousand years, how temperatures are measured, etc. This is all sadly misguided. The real issue is not temperature, but heat. If temperatures haven't changed in 15 years, but most of the volume of ice in the Arctic has melted over the same period, that is evidence of global warming.

For this reason, skeptic arguments that begin 'it was warmer in the past...' are irrelevant. Whatever set of solar activity, orbital forcings, and random variation caused the Medieval Warm Period (MWP) is not active today, and if it were, things would be much worse. We've reached the temperature extremes we have without their help.

The Little Ice Age and MWP were extremes, and temperatures reverted to the trend afterward. The point of AGW is that we've changed the trend. The new trend in temperature is not an unexplainable random variation that will pass, allowing the world to cool. Is is explainable by and attributable to AGW. It is up, not down.

Catastrophic?

All of the different forcings interact with each other, making it difficult to tease apart the effects of a single change. The effect of rising CO2 is itself limited. What has alarmed climate scientists is the effect of the interactions. Remember our aside about Mars. The erstwhile Terraformers only have to supply enough GHG to warm the icecaps. After that, the effect of the CO2 ice re-entering the atmosphere warms the planet much more. This is an example of a positive feedback. More CO2 in the Martian atmosphere captures more heat and raises the temperature which melts more CO2 ice, resulting in more CO2 in the atmosphere.

These same sorts of positive feedbacks can occur on Earth. As an example, warm water can hold less dissolved gas than cold water. If the oceans warm, they will give up gases to the atmosphere, including CO2. The added CO2 will warm the oceans even further. It doesn't matter which came first, once the loop starts working in a certain direction, it is self reinforcing. You need an external event, such as a volcano erupting or an orbital wobble, to stop the cycle and send it in the opposite direction. And yes, it works just as easily in the opposite direction. Colder water absorbs more gas, pulling it out of the atmosphere, leaving less to warm the planet.

Unlike measuring the IR absorption spectra of a gas, accounting for all the forcings and interactions is difficult. Climate models have been built that try to account for them all, but there are extreme difficulties modeling an entire planet in enough detail to get a clear view of what is important.

This question of what is the multiplier effect of the CO2 is the place where the science is the least settled.

So what?

In an argument about global warming, skeptics often go through stages of acceptance, similar to the 5 stages of grief. From an opening position denying GW is happening at all, they will be forced by the preponderance of evidence to agree that yes, GW is happening, and yes, it is anthropogenic. Perhaps the multiplier really is strong. But so what? Isn't GW really a good thing?

Basically, no. Our economic and ecological systems are tuned to a certain set of parameters. The growing season is this long at this latitude. Rain falls here in such and such an amount. The tides are this high.

Changes in climate eventually manifest as changes in weather. For the Northern Hemisphere, one serious way that will happen relates to the jet stream.

The jet stream is a band of fast moving air circling the North Pole. It marks the boundary between cold polar air and more temperate air. The recent melting of the Arctic ice has weakened the jet stream (By weakening the contrast between polar and mid-latitude air temperatures), leading to it becoming unstable. In terms of weather, this has led to the phenomenon of "blocking highs". Weather systems develop and then stay put, baking or freezing a region instead of moving on. A pattern called 'Warm Arctic, Cold Continents' (WACC) develops in the winter. Snowmageddon, Snowpocalypse, the killer cold in Europe in 2011 are all examples.

Anyone who has seen a century of growing season maps knows that the effect of AGW is real. It has a cost, in forcing farmers to plant new crops, learn new methods. It has a cost in contributing to extinction of species by reducing and relocating habitats. AGW is producing changes in decades that used to take thousands of years.

Besides changes in weather patterns, sea level rise is usually listed as a large danger stemming from AGW. If significant amounts of land based ice were to melt, there could be a rise in sea levels. The sources of this would have to be the Greenland ice sheet in the Northern Hemisphere, and the Antarctic continental ice sheet in the south. Mountain glaciers, while disappearing quickly, are not an appreciable source of water compared to these large ice sheets.

Ice sheet melting and sea level rise will happen much more slowly than other, weather related changes. However, they are also irreversible within the span of human technologies now available. A rise in sea level of one foot on average by the end of the century seems possible. It doesn't sound like much, but could affect millions of people who live in low lying areas such as Bangladesh (or Florida). The use of an average number obscures the effect on storms and storm surges. We have built the infrastructure of our cities for a certain sea level, and rebuilding for a different sea level will cost a lot of money. The storm surge of Hurricane Sandy brings this lesson home.

AGW is just the latest in a series of challenges to the rule of law, and the extension of legal principles across national borders to address "tragedy of the commons" issues. The tragedy of the commons refers to the situation in an English village where the 'commons' was grassland owned by everyone and no-one. As a result, everyone felt free to graze their sheep there. Overuse leads to destruction and the resource becomes useless to everyone.

Acid rain and the ozone hole are previous examples of nations working within the legal system to handle injuries to the commons of the atmosphere.

Skepticism

Instead of addressing the range of policy responses to AGW, many skeptics would prefer to argue that the problem does not exist, has a source other than man, cannot be stopped, or is a good thing anyway. We have shown above that the problem does exist, and its source is man-made. Therefore it is up to us to try to ameliorate the effects of our own actions.

Reading the comments at a popular skeptical website such as WattsUpWithThat, it is clear to me that there are several common themes among skeptics. They distrust national governments and distrust the idea of cross-national coordination. They fear 'one world government', and see a conspiracy to impose the UN as a world government (just a front for Russian communists, of course) through the ruse of GW scare-mongering. They deeply resent any possible restriction on personal privilege, or additional cost associated with specific behaviors.

The result is that skeptics misuse their energies arguing against the facts instead of creating useful alternatives to possible policies they dislike. If you don't like a blanket carbon tax, push for tax credits for hybrid vehicles. Replacing a gas guzzler with a hybrid is the easiest and most effective thing an individual in the developed world (specifically America) can do to reduce their carbon footprint - their personal contribution to AGW. Raise taxes on beef products to recognize the high cost of raising cattle. In other words, make fighting GW a personal choice, in line with the philosophy of personal choice and taking personal responsibility for our actions.

Unfortunately, skeptics prefer to demonize certain high profile scientists such as Michael Mann, Phil Jones, James Hansen, and Kevin Trenberth. The skeptical community has bought into the same doubt-casting that the tobacco industry used to fight the scientific evidence that smoking causes cancer. Big Oil has turned to the same set of spin doctors as Big Tobacco used, such as The Heartland Institute, and reuses much of the same playbook, such as setting up fake 'grassroots' concern organizations.

Hurricane Sandy

As a very topical point, many people have speculated about the connection between Hurricane Sandy and global warming. Kevin Trenberth's opinion piece in The Scientist lays out the facts as they relate to Sandy in particular. Minnesota Public Radio's web site has also carried two recent pieces on Sandy hitting New Jersey, the coming winter weather and climate change - specifically the melting of Arctic sea ice.

Hurricanes form over warm tropical waters in the Atlantic near Africa. They become self-sustaining, as long as they can draw energy from ocean waters above 25C in temperature. If the ocean in the North Atlantic stays warmer than 25C even in high latitudes, hurricanes can sustain themselves farther north than 'normal.'As an example, when Sandy crossed over the Gulf Stream at the level of the Carolinas, the storm intensified as it pulled heat out of water that was 27C (80F!). Obviously, if the oceans contain more heat, they can generate more tropical storms. This can manifest as storms earlier in the season, later in the season, or more powerful storms.

Instead of focusing on a single storm such as Sandy, it is more appropriate in my opinion to look at an index of the total power of all the storms in a season. One such measure is the accumulated cyclone energy of the entire Atlantic. While there is a clear upward trend in the raw data, there are also confounding influences, such improved data gathering over time. However the overall implication is clear that tropical storms are increasing due to more available energy in the oceans.

Hurricanes are engines for taking heat from the ocean, raising it to the top of the atmosphere, and allowing it to radiate away into space. As such, they are an alternative to a gradual rise in global temperature, just as melting ice is an alternative.

Summary

• Global warming is happening. The planet is holding more heat from the sun than it used to. This heat will eventually have visible effects, such as melting Arctic ice, powering more, and more powerful, storms, and changing weather patterns.
• Global warming is an inevitable consequence of adding CO2, a powerful and long-lasting greenhouse gas, to the atmosphere.
• Global warming is caused by our activities. Burning fossil fuels has several effects on the atmosphere, depending on what in specific is being burned. Soot and aerosols may briefly cause clouds that cool the planet, but CO2 and soot falling on ice warm the planet. The strongest and most consistent effect of the use of fossil fuels in the rise of CO2 in the atmosphere. (Tropical deforestation also releases CO2, and causes other negative effects that warm the planet.)
• Anthropogenic global warming has reversed the long term climactic trend towards cooler climate. This is happening faster than the global infrastructure can react without significant costs.
• Measures of heat in the environment continue to rise, even while temperature records show volatility.
• While much of the debate on AGW has focused on temperature records and proxies for temperature records, these records involve the mixed effects of multiple drivers. The one driver that correlates best with temperature change is CO2 rise. 'Cycles' and 'natural variation' are often cited by skeptics as the cause of the temperature record, but all such sources have already been included and found wanting as explanatory variables.
• If El Nino, cosmic rays, or the Sun were responsible for global warming we would still have a responsibility to ameliorate what we can. Even more so when the cause is our own lifestyle choices. This is a fixed principle of our moral and legal systems.
• The forward looking modeling of climate change shows that the effect of CO2 rise will have a multiplier effect. What that multiplier is is not clear at this time, but skeptics aren't helping to clarify the issue by constructing better models - they are tearing down the idea of trying to solve the problem at all.
• Many of the effects of AGW will take longer than a single human lifetime to manifest themselves. This is no excuse for inaction. Amelioration becomes harder and more costly, the longer we wait. Paper companies plant forests. Insurance companies invest in bonds. We know how to make long term decisions with the hope that our children's lives will be better - that is what brought us all to America. AGW is no different than other environmental challenges that we have become aware of over time.
• The miracle of compound interest is currently working against us, but that can be changed. Investing now in solutions will save enormous amounts in the future, not just of money but of human misery.
• The debate over global warming is a controversy manufactured by large corporations to protect themselves. These companies have co-opted the fears and political tendencies of individuals to protect themselves as corporate entities. Good hearted and open minded individuals of all political persuasions can, and should, participate in the policy debate based on a common set of facts. AGW is real, what we do about it is the question.