Climate Change: Past, Present, and Future Implications for Life on Earth

[aquitaine]

1. Why did the Earth's climate cool so much during the Cenezoic?
2. Will global warming (if nothing is done about it) cause the climate to return more or less to what it was before it? If so, how would that affect life as it is today (who loses and who benefits, for example will corals survive)?

 

[Wagmc]

1. I would ask "what was the sun doing at that time?"

2. Ignoring A) whether "global warming" is, in fact, occurring for the last decade and B) whether any of its causes are within man's control. Most species, including corals and polar bears, have survived quite nicely in temperatures significantly higher than today. No reason to think that they are not similarly adaptable today.

 

[chriscolose]

1. Why did the Earth's climate cool so much during the Cenezoic?
2. Will global warming (if nothing is done about it) cause the climate to return more or less to what it was before it? If so, how would that affect life as it is today (who loses and who benefits, for example will corals survive)?

Changes in solar activity is not the answer to your #1. If anything, stellar star evolution theory predicts slightly warmer temperatures now than during the beginning of the Cenozoic if only solar activity was being considered. The radiative forcing of solar increase over the Cenozoic is about 1 W/m^2 (Hansen et al 2008), which is only about 60% of the current carbon dioxide forcing. Like most deep-time climate problems, the answer is more so involved in changes in the chemistry of the atmosphere (CO2 especially) and the albedo of the planet. There are a few things going on: The best explanations I have seen involve the continental movement and distribution of landmass to higher latitudes which favors a climate that is prone to the glacial-interglacial cycles we are used to. Among this is increased weathering and reduction of CO2 concentration associated with growing mountain belts (Himalayas and Tibet, e.g., Garzione 2008) as well as changes in ocean heat transport (changes in temperature are not very large at the tropics and larger as you move toward the poles) and cloud cover (one mechanism of possible changes in cloud cover over the Cretaceous, and presumably any large scale deep-time climate shift, was presented in Kump and Pollard 2008 and is associated with changes in biological productivity under different temperature regimes inducing changes in cloud condensation nuclei). These mechanisms are generally well accepted to be important although their exact relative importance and possible other factors is not something which looks to be settled.

For #2, I think the question is a bit ill-posed, but the global temperature can easily revert back to Cretaceous type levels if all fossil fuel reserves are exhausted. The climate itself refers to more than just temperature, and we are also interested in regional temperature changes and not just global ones, so because the continental positions and ocean circulation patterns are different now (and because we're not going to lose all of Antarctica) and sea levels are different, then there is certianly going to be some differences between now and then, even with equivalent atmospheric CO2 levels. A transition to a planet with no Northern Hemispheric ice and much higher sea levels would likely take hundreds to thousands of years, but because the effect of our extra CO2 can last that long, the long-term outlook can easily be of similar magnitude to the type of climate shift that separates geologic time intervals. Obviously the biosphere is going to respond to changes in climate, and what generally matters more than the magnitude of the change is the rate of the change. We're changing the climate in a much faster pace than it does on its own through Milankovich cycles or longer-term continental and weathering changes. Some things are going to lose out and some things are going to benefit. Usually the more rapid and the larger the change, the more things lose.

Corals have "lost" big time in the past, but have recovered on geologic timescales, so people who argue that corals are in no danger because they've been around before are making little sense. Life survived asteroid impacts and the Permian as well. Life survived the oxygenation of the atmosphere by plants and other organisms, although this was catastrophic to anaerobic organisms. The point is not really whether "The Earth will survive" or if "life will survive" or even if things have survived in higher temperatures and are still around today. These simple connections are usually good talking points (generally for skeptics) but not very meaningful for application to today's climate change. You'll probably have to consult the IPCC WG2 or more detailed ecological reports for specific regional impacts or details about specific species.

 

[aquitaine]

Interesting, thanks.