Alasdair Wilkins in io9.com:
Climate change could have devastating consequences for much of the world's ecosystems, but at least one area might benefit. Ancient rain forests thrived during severe warming millions of years ago, helping to create today's species diversity.
56.3 million years ago, planet Earth was going through one of its hottest periods. Average temperatures were 3-5 degrees higher than they are today, and carbon dioxide levels in the atmosphere a whopping 2.5 times thicker than what we've got today. Both of those are fairly extreme, and it seems reasonable that life struggled during these times. But, at least in the rain forests, that wasn't the case at all.
This particular period of global warming was known as the Paleocene-Eocene Thermal Maximum. Around 56.3 million years ago, Earth's temperature started increasing, and it took only 10,000 years for carbon dioxide to reach such high levels. This remained the norm for the next 200,000 years. Despite these seemingly harsh conditions, many species thrived, as the diversity of rain forest species tripled during this period.