Can somebody cite a period when climate HASN'T changed?
It all depends upon your time scale and your definition of what constitutes "climate change". My understanding of the Earth's climate history is that most of it has been slow transitions, and that fast transitions have been disruptive to life forms, and particularly disruptive to larger life forms, on our planet.
In terms of the current debate, the primary concern is the potential rate of climate change, specifically temperature increase. Mankind's ability to adapt depends not only on how much the climate may change but how quickly that change occurs. I have no doubt mankind can handle an increase of 5 degrees Celsius over 5,000 years. This has been the average rate of the Earth's natural temperature rise after an ice age, based upon the known temperature history of the last 800,000 years. However, we may already be within a few degrees of the warmest the Earth has been over that 800,000 year span. An additional 5 degree Celsius temperature increase over the next 100 to 200 years, as predicted by many climate models, would be much harder to handle.
Thanks for your explanation. Having lived for many years in the US "southwest" (I have lived in 3 of the "4-corners" states), I have encountered several "pre-historic" situations where the only explanation for a sudden failure of a society or "civilizations". Years ago, park rangers at Mesa Verde National Park and among the Pueblo, anthropologists at digs, and just plain old amateur hobbyists expressed that it didn't make sense that a seemingly thriving society just suddenly either disappeared or left their infrastructure, apparently going from the height of development to nothing in a period of only a very few decades. Evidence was that food sources disappeared or water was suddenly not sufficiently available after being there for centuries. they all said that such swift changes were unexplainable. Well, it's now apparent that such changes do and have occurred every few centuries. Such changes are only really evident in areas or zones that are on the fringe of being able to support larger population densities, like the southwest. Yes, these areas cover only a small fraction of the earth's surface, and may be isolated cases. But the evidence is there. Arguably, these conditions occurred in part because the society became too large or too successful, adding demand on the environment, and were at least in part man-made, but not unique to our current society and development.
What I find most interesting about this discussion on Global Warming/Climate Change, is that nobody seems to recognize that we are in an inter-glacial period that started about 11000-12000 years ago, when at least half of the Northern Hemisphere was covered with ice sheets up to a mile thick.
The natural expectation would be that the temperatures of the Earth SHOULD show a warming TREND, regardless of yearly temperature fluctuations, and study of any graphs will show that that is exactly what is happening. as the planet warmed, the rate of this trend became APPROACHED a relatively straight line. As for rapidity of climate changes, there have been periods where climate has changed significantly in less than a decade; I may be wrong, but the Younger-Dryas extinction relatively soon after the end of the last glaciation was one of these extreme events.
Another problem with emphasizing CO2 concentration as responsible for Global Warming/Climate Change is considering correlation to be causation. While CO2 may be involved, few people seem to consider the changes in land use not only since the start of the Industrial Revolution, but since the final evolution of homo sapiens. Just think of how many forests have been denuded, or actually just literally cleared not only since the industrial Revolution, but by thousands of years of human existence. Ironically, some of the sources of energy to replace fossil fueled power plants, particularly solar and wind farms, would just exacerbate the problem.
Hi, Steven Brecht. Even during the Last Glacial Maximum, which occurred about 24,500 BCE, nowhere near half of the northern hemisphere was covered in ice sheets up to a mile thick. Around 11,000 years ago, the northeast part of Canada and Greenland were covered in glaciers, with strips of glaciers elsewhere in North America. (http://www.historymuseum.ca/cmc/exhibitions/aborig/fp/fpz2f13e.shtml)
Human civilization started with the Holocene, ~12,000 years ago. In 2013 it was hotter than 90% of the Holocene (http://science.sciencemag.org/content/339/6124/1198).
Climate scientists are smart enough to know that climate changed in the past and that land use changes could affect climate. In fact, they use information about past climate change to learn about what is affecting today's climate and what future effects might be. There is a whole branch of paleoclimatology that focuses on previous rapid changes in climate.
There are multiple lines of evidence that point to human-caused CO2 resulting in climate change. An extremely partial list:
We know from physics that increasing the concentration of CO2 and other greenhouse gases in the atmosphere affects the earth's energy balance
We know that levels of CO2 and other greenhouse gases are high and rising: CO2 is the highest it's been in the last 780,000 years (for the last 400,000 years see http://cdiac.ornl.gov/trends/co2/graphics/vostok.co2.gif, for recent measurements see http://blog.ucsusa.org/wp-content/uploads/2014/03/mlo_full_record.png)
We know that global temperatures are increasing (http://data.giss.nasa.gov/gistemp/graphs_v3/)
We know the temperature increase is from greenhouse gases because if it wasn't, the vertical temperature profile of the atmosphere would look different (http://onlinelibrary.wiley.com/doi/10.1029/2004JD005075/full)
We know that the increase in greenhouse gases in the atmosphere is from combustion of fossil fuels, because it isn't from other sources and they're depleted in 13C (http://www.realclimate.org/index.php/archives/2004/12/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated/)
We also know that if we exclude anthropogenic sources of greenhouse gases, the climate models don't work (and they do work very well) (also from Santer et al 2004)