The suggestion is that it was massive underwater volcanism that led to the anoxic oceans and mass extinctions of 95 million years ago.
If true, I suppose it is half-way encouraging that high levels of atmospheric CO2 alone might not lead to anoxic oceans. But, this is not something I feel particularly inclined to run the risk on.
The Nature article I linked to a couple of posts back argued that the world could reach 1000 ppm pretty easily. Here's the relevant section (if you are really lazy, just read the parts I have put in bold):
The goal of climate mitigation is to avoid dangerous human-caused impacts, which science suggests would mean limiting total warming to 2 °C above preindustrial temperatures. In turn, this would require keeping atmospheric concentrations of carbon dioxide below 450 parts per million (p.p.m.). According to the IPCC's Fourth Assessment Report in 2007, model studies based on our current understanding of climate–carbon-cycle feedbacks suggest that to stabilize carbon dioxide levels at 450 p.p.m. could require that cumulative emissions over the twenty-first century reach only about 490 gigatonnes of carbon (GtC), which equates to less than 5 GtC per year1.
Similarly, stabilizing atmospheric carbon dioxide levels at 1,000 p.p.m. would require cumulative emissions this century of only about 1,100 GtC. In other words, if annual emissions average 11 GtC this century, we risk the real, terrifying prospect of seeing 1,000 p.p.m. carbon dioxide in the atmosphere and a 'best estimate' warming of a staggering 5.5 °C by the end of the century.
Carbon emissions from the global consumption of fossil fuels are currently above 8 GtC per year and rising faster than the most pessimistic economic model considered by the IPCC2. Yet even if the high price of energy from fossil fuels and power plants combines with regional climate initiatives to slow the current rate of growth somewhat, we will probably hit 11 gigatonnes of carbon emissions per year by 2020.
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