As might be expected, higher temperatures and more CO2 lead to
greater plant diversity.
One such episode in Earth's history occurred 56.3 million years ago and is called the Palaeocene–Eocene Thermal Maximum (PETM). Within 10,000–20,000 years, the world warmed by 3–5 °C and atmospheric carbon dioxide doubled to around two and a half times the levels we see today. These unusually warm conditions lasted for around 200,000 years.
Not wishing to misstate things:
However, Guy Harrington, a palaeobiologist at the University of Birmingham, UK, warns that any positive effects on plant diversity could be cancelled out if temperatures rise too quickly for plants to adapt. "It's the rate — how fast you're turning up the heater — that's the most important thing," he says.
Not to mention that these effects "may also be limited to the tropics. That said, it concludes:
Jaramillo believes that there is a more pressing threat to the diversity of tropical rainforests. "Deforestation is the real enemy," he says, "not the increase in temperature and carbon dioxide."
This sounds a lot like Lomborg's eminently sensible maxim: If Polar Bears are what we're worried about, we could quit shooting them.
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