Let's clarify what global warming is, the actual mechanism of it. It's not that difficult to understand, and will reveal to you the severity of the situation we're in. As a thorough primer on the subject, I again refer you to James Hansen's Storms of My Grandchildren (although, his final conclusions do not adequately portray the emergency of the situation).
So, what causes global warming? The answer may not be what you're expecting me to say. Energy imbalance in the whole sun-earth system results in global warming or in global cooling. Basically, the Earth is like a rock next to a campfire. If the rock absorbs more heat from the fire than it can simultaneously radiate into the air around it, the rock will grow hotter, until the point where incoming energy and outgoing energy balance each other. If the rock gives off more heat than it absorbs from the fire, it will cool down, until balance is reached. The Earth behaves exactly the same way.
There are three basic ways to increase the energy imbalance between the Earth and the sun. First, the sun could send more energy toward the Earth – that is, it could increase its irradiance. Second, the reflectivity of the Earth – its albedo, or shininess – can be altered. If the Earth reflects more heat directly back into space, it absorbs less heat, and vice versa. Third, the insularity of the Earth can be changed: heat can be held against the Earth for a longer time (like body-heat under your winter coat), creating an imbalance in the amount of energy coming in from the sun, and the amount radiating out into the deep night of space. (There is a fourth way, which is to bring the Earth and the sun closer to each other, but astronomical measurements show that is not happening enough to cause the warming measured on Earth.)
Our best measurement of the Earth's net energy imbalance has it somewhere in the range of 0.5 - 0.75 Watts per square meter. The location of this warming (heating in the lower atmosphere, cooling in the stratosphere) is the signature of energy imbalance caused by insularity, not greater solar irradiance.
Point-six Watts probably sounds like a small amount of energy. It is equivalent to one dim Christmas tree light-bulb on each square meter of the Earth's surface. However, when you consider the total brightness of all those light-bulbs together, the total magnitude of that energy, you can realize that it is actually very large.
By comparison, the Energy imbalance that triggered the Earth's entry into the most recent ice age (and more importantly, its rapid and chaotic exit from it) was a measly 0.1 Watt/m2. How on Earth did that small jolt push the Earth into one of the biggest climate shifts ever experienced? The likely reason, revealed by the paleoclimate evidence, is that this miniscule energy imbalance was enough to trigger major feedback cycles that amplified the initial effect. The most major of those feedback loops was change in atmospheric carbon dioxide levels and change in albedo due to transforming polar ice sheets.
Looking specifically at the end of the last ice age, we see that the 0.1 Watt/m2 (resulting from slight changes in the Earth's axis of rotation, which increased the level of solar radiation reaching the northern hemisphere) was enough to kickstart the rapid melting of polar ice, which then reinforced its own melting, leading to the rapid disintegration of the icecaps. The oceans warmed enough to release significant amounts of CO2 into the air, which drove us into our current age of unusually stable climate (after an initial spike).
Now, I'm going to tell you something that should shock you. Guess the amount of the current energy balance from our loading the atmosphere with carbon dioxide through burning fossil fuels (are you ready for this?). Not 0.1 W/m2. Not 0.5 W/m2. Not even 1 W/m2.
Friends, the energy imbalance from burning fossil fuels to date is 3 W/m2. Wow. That is 30 times the size of the force that previously changed the planet from a world of ice into the world we know.
In fact, there is a time in Earth's history that more closely resembles what is happening today. 50 million years ago, India was plowing northward through the future Indian Ocean, toward the Asian landmass. The subcontinent was moving at such a fast rate that friction with the ocean floor caused CO2 that was trapped in the Earth's crust to be released in large volumes. So goes the theory. This is the closest natural approximation we have to the kind of situation we're in, where humanity has extracted carbon that was in long-term, stable storage in fossilized deposits, and converted it to CO2 in the atmosphere.
So, what happened 50 million years ago? A sophisticated proxy of the temperature at that time (an indirect index recorded in the ratio of isotopes of oxygen and carbon) reveals that there was a dramatic increase in global temperatures, as much as 12-15º C. Alaska was tropical, and hosted crocodiles. At a certain point, early on in this temperature crescendo, 3000 million tons of methane hydrate, frozen on the sea floor, suddenly melted, and the global average temperature spiked for 200,000 years. There were no polar or mountain icecaps anywhere on Earth, and ocean levels were very much higher than they are now. Not only that, but ocean currents actually appear to have reversed direction for about 40,000 years. Suffice it to say, the Earth of the Eocene Optimum would seem like an alien world to today's humanity.
Our contemporary carbon pollution of the atmosphere is occurring at roughly 5 times the rate that created the Eocene Optimum. The ocean is already more acidic from industrial carbon dioxide than it ever was in the Eocene.
We begin to have some sense of the scale of the energy imbalance that we are engendering. Strangely enough, somewhere between 1/3 and 2/3 of that 3 W/m2 imbalance that we are forcing with greenhouse gas emissions is masked by reflective aerosols that we are simultaneously emitting as industrial pollution – aerosols that block sunlight, and therefore artificially cool the planet. As we tighten regulations on other pollutants, like sulfur-oxides, without regulating carbon dioxide sufficiently, we stand the risk of revealing the true impact of our contribution to global warming. Because these aerosols are only suspended in the atmosphere for a short time, if we were to stop emitting aerosols completely today, within a few weeks or months, we could see the net energy imbalance of the planet double – and that would be catastrophic.
There is one thing worth mentioning, as well. In the time it takes to equilibrate an energy imbalance, other reinforcing feedback loops can be triggered which cause the balance point to shift, and the new forcing factor may not be something we can control. For instance, we certainly don't have any way currently to keep 5,000 million tons of methane hydrates from melting on the sea floor, if they start to do so en masse. There are indications that we have so far overlooked certain potential feedback loops that could have very significant impact on the scale of global warming. So far, for example, the Earth's landmasses have continued to absorb CO2 at the same proportion, year after year, even though the total load of CO2 output into the environment from human industry has been increasing. No one thinks that this surprising pattern can persist indefinitely. In fact, an ongoing study conducted in Colorado, comparing plots of Earth subject to natural conditions with plots of Earth which have electric heaters placed over them, reveals that, as temperatures rise, the soil itself can suddenly switch to become a source of CO2, rather than a carbon sink. When it does, we can expect that the release from the soil will yield a doubling of CO2 in the atmosphere.
This, then is the emergency. We are forcing the planet warmer at a speed and intensity unmatched by natural mechanisms of climate change. We're not even in the same ballpark of effect. Though our actions may seem routine to us, they are actually unprecedented in their impact, and will assault the stability upon which civilization has depended with unbelievable force. Every once in a while, there is an event on Earth that takes only a few hours, or days, or even a handful of years to happen, but whose consequences play out for millions of years thereafter, and fundamentally shape the history of the Earth. The comet impact that extinguished the dinosaurs and transformed the planet's climate was such an event. We are now well within the theatre of yet another one, though most people don't realize it, let alone accept that they are the principal actors on the stage. In a unique moment that challenges the human mind like nothing else in our species' collective experience, we are asked to accept that the choice among alternate histories, which will unfold for millions of years to come, will be decided in the next five years.
(Written by Gavain U'Prichard)
(Written by Gavain U'Prichard)