It happened before. Well, not quite this rapidly, but about 55 million years ago (to geologists, the recent past), Earth experienced an explosive rise in temperature. Through the investigative science of paleontology, we can reconstruct a world where tropical flora flourish at the poles and the world is rocked by mass die-offs - that is, the world that the Intergovernmental Panel on Climate Change predicts as the "worst-case scenario" by 2100.
Six degrees Celsius may not sound like a catastrophe, but that only tells part of the story. Most of the world can at least apportion its heat, but warmer currents converge on the poles with a particularly vicious effect. Normally, the white icecaps reflect much of the sun's energy (objects appear to be a certain color because of the wavelength of light they reflect back at us, and white light is what we see when every color is reflected back). However, as they melt, there is less white and less reflection of solar energy back to space, resulting in accelerating melting.
All this warmer, fresh water floods into the global ocean and helps to heat it up even more than the sun beating down on it could do alone. At this point, the temperature rise can be tracked in the type of oxygen that microscopic marine drifting organisms use to build their chalky exoskeletons. Elements like oxygen come in multiple versions, called isotopes, which are the same element but are sometimes heavier or lighter, and this can be examined in the fossil record.
Normally, the drifters 'prefer' the heavier version of oxygen, but when the ocean heats up, the lighter version is more accessible because it gets moved around more easily. Fossil drifters show that around 55 million years ago, there was a spike in light oxygen being taken up by drifters, and thus a spike in temperature.
The Paleocene-Eocene Thermal Maximum (PETM), as scientists call it, was a shocker to the Earth. Up to one-half of all species of foraminifera (a kind of drifter that makes its skeleton of chalk and usually lives on the seafloor) went extinct, triggering a probable collapse of the marine food chain. In parts of rural Montana you can still clearly see a sudden layer of red clay in the sedimentary record. That clay is important because it shows what's missing: usually, the skeletons of foraminifera blanket the sediment, but in their absence, the clay is all that's left.
A lot of these effects are not in fact due to the rising temperatures, but instead are another side effect of the injection of carbon dioxide, or CO2, into the atmosphere. When plants are exposed to heightened CO2 levels, they are much more productive - but they lose out in protein payoff per kilogram! Land animals shrank in size, because when protein is hard to come by, natural selection favors an animal that can survive on less. The fossil record attests to everything from dwarf condylarths (strange mix-'n'-match mammals that have been called "sheep in wolves' clothing") to even insect burrows getting skinnier. Meanwhile, nutritionally deficient tropical plants expanded their range, pushing out local competition and disrupting ecosystems around the world.
The exact cause of the PETM is still a mystery, although several very knowledgeable fingers have been pointed at a release of methane locked up in ice at the bottom of the sea. Methane, like carbon dioxide, is a greenhouse gas that traps heat in the Earth's atmosphere and helps to keep the Earth hotter. Both of these gases are currently being emitted at record levels by human activity, and correspondingly the Earth has been heating up, as a global average, to a record high. It's happened before, and it might happen again - but this time it would be our fault, and our problem.
Some might have noticed that the Earth seems to have gotten over the last event pretty much fine. Why should this be any different, the hecklers in the audience demand. Last time, organisms moved and populations adapted. The Earth's inhabitants are usually good at that. Not us. As a species, we humans want to live wherever we want, eat whatever we want, and live our lives however we want. That just won't work. We must react or die. According to the latest data, the concentration of CO2 in the atmosphere hit almost 400 parts per million. The Intergovernmental Panel on Climate Change has endorsed 350 parts per million as a sensible upper limit. If we want to avoid an apocalyptic scenario, emissions must be reduced and minds shifted toward the ultimate goal of saving everything we know and love on this planet for both ourselves and future generations.
Luckily, there is still hope. That hope is you. That hope is in lessening our dependence on burning fossil fuels for energy. That hope is in conserving our resources and ensuring that there is enough to go around. That hope is in lobbying our governments and standing up to our corporations. That hope is in human resilience and ingenuity that I trust will manifest itself, but that hope is also the little things we do every day. Our little choices - the decision to turn off that light or forgo that hamburger or save that plastic bag - constitute a sum greater than their parts. They constitute the solution. Now that we understand the past and can more clearly see the possibilities for the future, there is one part left: to take the present into our hands.