Geologists have found crocodile and lemur fossils in the arctic. At some time in the past, the Earth was much warmer than it is today and tropical vegetation extended from the arctic to the Antarctica. How did this happen?
About 65 million years dinosaurs dies out due to some combination of asteroid impact and volcanic eruptions. This set the stage for small rodent like mammals to occupy other evolutionary niches that were hitherto closed to them. But interestingly the overall climate of the world sis not change so much. This was the Paleocene Era. The Earth looked very different then. India still hadn’t liked up with Asia and the Himalayas were flat. At the other end of the world, South America was still linked with Antarctica. This had a profound impact on the ocean currents in the region as it separated the Atlantic and Pacific Oceans.
Then about 55 million years ago, something remarkable happened. Within a few years, the average temperature of the Earth rose by 5 degrees Centigrade. That doesn’t sound like much until you realise that the local temperature at some places changed by as much as 40 degrees Centigrade. This had a profound impact on the path taken by evolution. This was the trigger for the development of many of the animals we recognize today including whales, dolphins, porpoises and even apes who had been relatively solitary creatures till then started banding in large groups as their range expanded beyond the tropics.
What had happened to the world? Here is one of the leading theories today.
When living things die in the ocean, their bodies fall to the bottom. Here they are covered with layers of sediment as bacteria work to decompose them. As there is no oxygen there, the carbon in the bodies decays to form methane. The methane gas gets heated by the heat from the Earth’s interior and rises to the ocean floor. Here it comes in contact with the cold currents at the bottom of the ocean and under high pressure and low temperature it freezes to form nodules. Scientists have found and brought some of these frozen gas bubbles to the surface where they produce the characteristic bluish flame of methane.
Around 55 million years ago, something happened at the ocean floor causing the currents there to heat up. The methane started bubbling up to the surface. In all, it is believed up to 100 giga-tonnes of carbon was released into the atmosphere. Further, the greenhouse gas due to methane is 27 times worse than carbon dioxide.
Of course, there are natural processes that worked to restore the balance of carbon in the atmosphere. In all, it took 200,000 years for the climate to return to something like its previous levels.
Why is this relevant? It is believed that by the end of the century, humans would have pumped 5% as much carbon into the atmosphere as was released at the end of the Paleocene. It is worth remembering that it could take thousands of years for the climate to recover.