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And what precisely does man have to do with this? Or better, what can or should man do about it seeing as temperatures seem to be returning to where they were 2000 years ago and therefore "ecological alteration" and "species distributions" are simply returning to their nominal state in response to conditions prevalent before "20th Century warming".

First, mankind's activities may have something to do with the current rate of warming. We'll be more sure of that (or not) in a few years. Second, the warming rate over the 20th century, 0.6 C, is faster by about 2x than any century in the past 2000 years. The "squeezed" nature of your graph doesn't show this very well. Natural ecosystems can adapt to a temperature rate of change maximum of about 2 C/century, which may be (note the qualification) the rate of temperature rise that has taken place since the mid-1970s. So, if mankind's activities are responsible for the current rate of temperature change, then the possible of ecosystem collapse, rather than adaptation, exists. I think the best estimates of the rate of temperature change in the 21st century are about 2-3 C, and a very recent paper first-authored by Patrick Michaels appears to support my opinion. So the 21st century could be a period where many ecosystems will be significantly stressed by rapid climate change.

First, mankind's activities may have something to do with the current rate of warming.

Let's put a number to that "may" shall we?

http://www.clearlight.com/~mhieb/WVFossils/greenhouse_data.html

Just how much of the "Greenhouse Effect" is caused by human activity?

It is about 0.28%, if water vapor is taken into account-- about 5.53%, if not.

This point is so crucial to the debate over global warming that how water vapor is or isn't factored into an analysis of Earth's greenhouse gases makes the difference between describing a significant human contribution to the greenhouse effect, or a negligible one.

Water vapor constitutes Earth's most significant greenhouse gas, accounting for about 95% of Earth's greenhouse effect (4). Interestingly, many "facts and figures' regarding global warming completely ignore the powerful effects of water vapor in the greenhouse system, carelessly (perhaps, deliberately) overstating human impacts as much as 20-fold.

Water vapor is 99.999% of natural origin. Other atmospheric greenhouse gases, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and miscellaneous other gases (CFC's, etc.), are also mostly of natural origin (except for the latter, which is mostly anthropogenic).

Human activites contribute slightly to greenhouse gas concentrations through farming, manufacturing, power generation, and transportation. However, these emissions are so dwarfed in comparison to emissions from natural sources we can do nothing about, that even the most costly efforts to limit human emissions would have a very small-- perhaps undetectable-- effect on global climate.

Role of Atmospheric Greenhouse Gases
(man-made and natural) as a % of Relative
Contribution to the "Greenhouse Effect"

Based on concentrations (ppb) adjusted for heat retention characteristics	Percent of Total	Percent of Total --adjusted for water vapor
Water vapor	-----	95.000%
Carbon Dioxide (CO2)	72.369%	3.618%
Methane (CH4)	7.100%	0.360%
Nitrous oxide (N2O)	19.000%	0.950%
CFC's (and other misc. gases)	1.432%	0.072%
Total	100.000%	100.000%

Anthropogenic (man-made) Contribution to the "Greenhouse
Effect," expressed as % of Total (water vapor INCLUDED)

Based on concentrations (ppb) adjusted for heat retention characteristics	% of All Greenhouse Gases	% Natural	% Man-made
Water vapor	95.000%	94.999%	0.001%
Carbon Dioxide (CO2)	3.618%	3.502%	0.117%
Methane (CH4)	0.360%	0.294%	0.066%
Nitrous Oxide (N2O)	0.950%	0.903%	0.047%
Misc. gases ( CFC's, etc.)	0.072%	0.025%	0.047%
Total	100.00%	99.72	0.28%

. Second, the warming rate over the 20th century, 0.6 C, is faster by about 2x than any century in the past 2000 years. The "squeezed" nature of your graph doesn't show this very well.

Really? lets take a another look:

Lets see, 0.6C^{^o}/last hundred years. Looks pretty clear from the last hunded to year 2000 to me.

0.6 C, is faster by about 2x than any century in the past 2000 years.

Stange I see several periods having changes of much greater than 0.6C^{^o}/century throughout the above graph.

900-1000AD looks to contain more than 1.8C^{^o}/Century change, three time the quoted 0.6C^{^o};
1200-1300 looks to contain more than a 0.9C^{^o}/Century change, three time the quoted 0.6C^{^o};
1400-1500AD looks to contain more than 1.2C^{^o}/Century change, twice the quoted 0.6C^{^o}

Natural ecosystems can adapt to a temperature rate of change maximum of about 2 C/century, which may be (note the qualification) the rate of temperature rise that has taken place since the mid-1970s.

So, if mankind's activities are responsible for the current rate of temperature change, then the possible of ecosystem collapse, rather than adaptation, exists.

Effect," expressed as % of Total (water vapor INCLUDED)

Based on concentrations (ppb) adjusted for heat retention characteristics	% of All Greenhouse Gases	% Natural	% Man-made
Water vapor	95.000%	94.999%	0.001%
Carbon Dioxide (CO2)	3.618%	3.502%	0.117%
Methane (CH4)	0.360%	0.294%	0.066%
Nitrous Oxide (N2O)	0.950%	0.903%	0.047%
Misc. gases ( CFC's, etc.)	0.072%	0.025%	0.047%
Total	100.00%	99.72	0.28%

I think the best estimates of the rate of temperature change in the 21st century are about 2-3 C, and a very recent paper first-authored by Patrick Michaels appears to support my opinion.

This Patrick Michaels ?

http://www.cato.org/pubs/pas/pa-329es.html

Latest Science Debunks Global Warming Hysteria

by Patrick J. Michaels

Patrick J. Michaels, a professor of environmental science at the University of Virginia, is a senior fellow in environmental studies at the Cato Institute.

Executive Summary

The national media have given tremendous play to the claims of Vice President Al Gore, some federal scientists, and environmental activists that the unseasonably warm temperatures of this past summer were proof positive of the arrival of dramatic and devastating global warming. In fact, the record temperatures were largely the result of a strong El Niño superimposed on a decade in which temperatures continue to reflect a warming that largely took place in the first half of this century.

Observed global warming remains far below the amount predicted by computer models that served as the basis for the United Nations Framework Convention on Climate Change. Whatever record is used, the largest portion of the warming of the second half of this century has mainly been confined to winter in the very coldest continental air masses of Siberia and northwestern North America, as predicted by basic greenhouse effect physics. The unpredictability of seasonal and annual temperatures has declined significantly. There has been no change in precipitation variability. In the United States, drought has decreased while flooding has not increased.

Moreover, carbon dioxide is increasing in the atmosphere at a rate below that of most climate-change scenarios because it is being increasingly captured by growing vegetation. The second most important human greenhouse enhancer -- methane -- is not likely to increase appreciably in the next 100 years. And perhaps most important, the direct warming effect of carbon dioxide was overestimated. Even global warming alarmists in the scientific establishment now say that the Kyoto Protocol will have no discernible impact on global climate.

So the 21st century could be a period where many ecosystems will be significantly stressed by rapid climate change.

I woulde say you have alot of very big "If"s and "could be"s to get over before a "will be" has much meaning as regards any contribution that mankind may have on the climate or effects thereof.