We know it%26#039;s warming, and we%26#039;ve measured how much:
http://www.epa.gov/climatechange/science...
Scientists have a good idea how the Sun and the Earth%26#039;s natural cycles and volcanoes and all those natural effects change the global climate, so they%26#039;ve gone back and checked to see if they could be responsible for the current global warming. What they found is:
Over the past 30 years, all solar effects on the global climate have been in the direction of (slight) cooling, not warming. This is during a very rapid period of global warming.
http://news.bbc.co.uk/2/hi/uk_news/62902...
http://www.pubs.royalsoc.ac.uk/media/pro...
So the Sun certainly isn%26#039;t a large factor in the current warming. They%26#039;ve also looked at natural cycles, and found that we should be in the middle of a cooling period right now.
%26quot;An often-cited 1980 study by Imbrie and Imbrie determined that %26#039;Ignoring anthropogenic and other possible sources of variation acting at frequencies higher than one cycle per 19,000 years, this model predicts that the long-term cooling trend which began some 6,000 years ago will continue for the next 23,000 years.%26#039;%26quot;
http://en.wikipedia.org/wiki/Milankovitc...
http://www.sciencemag.org/cgi/content/ab...
So it%26#039;s definitely not the Earth%26#039;s natural cycles. They looked at volcanoes, and found that
a) volcanoes cause more global cooling than warming, because the particles they emit block sunlight
b) humans emit over 150 times more CO2 than volcanoes annually
http://volcano.und.edu/vwdocs/Gases/man....
So it%26#039;s certainly not due to volcanoes. Then they looked at human greenhouse gas emissions. We know how much atmospheric CO2 concentrations have increased over the past 50 years:
http://en.wikipedia.org/wiki/Image:Mauna...
And we know from isotope ratios that this increase is due entirely to human emissions from burning fossil fuels. We know how much of a greenhouse effect these gases like carbon dioxide have, and the increase we%26#039;ve seen is enough to have caused almost all of the warming we%26#039;ve seen over the past 30 years (about 80-90%). You can see a model of the various factors over the past century here:
http://en.wikipedia.org/wiki/Image:Clima...
This is enough evidence to convince almost all climate scientists that humans are the primary cause of the current global warming. 53% 8 Votes 0% 0 Votes
http://ipcc-wg1.ucar.edu/wg1/wg1-report....
summarized at:
http://www.ipcc.ch/pdf/assessment-report...
There%26#039;s a lot less controversy about this is the real world than there is on Yahoo answers:
http://www.worldpublicopinion.org/pipa/a...
And vastly less controversy in the scientific community than you might guess from the few skeptics talked about here:
http://en.wikipedia.org/wiki/Scientific_...
http://www.sciencemag.org/cgi/content/fu...
%26quot;There%26#039;s a better scientific consensus on this [climate change] than on any issue I know... Global warming is almost a no-brainer at this point. You really can%26#039;t find intelligent, quantitative arguments to make it go away.%26quot;
Dr. Jerry Mahlman, NOAA
Good websites for more info:
http://www.nrdc.org/globalWarming/f101.a...
http://www.ucsusa.org/global_warming/sci...
http://www.realclimate.org
%26quot;climate science from climate scientists%26quot;
http://environment.newscientist.com/chan... 7% 1 Vote 0% 0 Votes
Skeptic argument:
%26quot;Global temperatures have been trending down since 1998. Global warming is over.%26quot;
Answer:
%26quot;At the time, 1998 was a record high year in both the CRU and the NASA GISS analyses. In fact, it blew away the previous record by .2 degrees C. (That previous record went all the way back to 1997, by the way!)%26quot;
%26quot;According to NASA, it was elevated far above the trend line because 1998 was the year of the strongest El Nino of the century. Choosing that year as a starting point is a classic cherry pick and demonstrates why it is necessary to remove chaotic year-to year-variability (aka: weather) by smoothing out the data. %26quot;
%26quot;Clearly 1998 is an anomaly and the trend has not reversed. (Even the apparent leveling at the end is not the real smoothing. The smoothed trend in 2005 depends on all of its surrounding years, including a few years still in the future.) By the way, choosing the CRU analysis is also a cherry pick -- NASA has 2005 breaking the 1998 record, though by very little.%26quot;
%26quot;Now, this is an excusable mistake for average folks who do not need the rigors of statistical analysis in their day jobs. But any scientist in pretty much any field knows that you cannot extract meaningful information about trends in noisy data from single-year end points. It%26#039;s hard to hear a scientist make this argument and still believe they speak with integrity in this debate -- seems more like an abuse of the trust placed in them as scientists. %26quot;
%26quot;It has stopped warming%26quot; is only supported by selecting a single year out of context and using a seven-year window to look at multi-decadal trends in climate. That%26#039;s a classic cherry pick.%26quot;
Mid century cooling? covered here
http://gristmill.grist.org/story/2006/11...
%26quot;None of the advocates of the theory of anthropogenic global warming claim that CO2 is the only factor controlling temperature in the ocean-atmosphere climate system. It is a large and complex system, responsive on many different timescales, subject to numerous forcings. AGW only makes the claim that CO2 is the primary driver of the warming trend seen over the last 100 years. This rise has not been smooth and steady -- nor would it be expected to be.%26quot;
%26quot;If you look at the temperature record for the 1990s, you%26#039;ll notice a sharp drop in %26#039;92, %26#039;93, and %26#039;94. This is the effect of massive amounts of SO2 ejected into the stratosphere by Mount Pinatubo%26#039;s eruption. That doesn%26#039;t mean CO2 took a holiday and stopped influencing global temperatures; it only means that the CO2 forcing was temporarily overwhelmed by another, opposite forcing.%26quot;
%26quot;The situation is similar to the cooling seen in the %26#039;40s and %26#039;50s. During this period, the CO2 warming (a smaller forcing at the time) was temporarily overwhelmed by by other factors, perhaps foremost among them an increase in human particulates and aerosol pollution. Pollution regulations and improved technology saw a decrease in this latter kind of emissions over the %26#039;60s and %26#039;70s, and as the air cleared, the CO2 signal again emerged and took over.%26quot;
%26quot;As the graph shows, in addition to aerosol pollution (the sulphate line), volcanic influences were increasingly negative during the period of global cooling, and solar forcing slightly declined. All forcings taken together and run through the model are a very good match for the observations.%26quot;
%26quot;Rather than confounding the climate consensus, mid-century cooling is actually a good test for the climate models, one they are passing quite convincingly.%26quot;
The medieval warm period covered here.
http://gristmill.grist.org/story/2006/12...
%26quot;There is no good evidence that the MWP was a globally warm period comparable to today. Regionally, there may have been places that exhibited notable warmth -- Europe, for example -- but all global proxy reconstructions agree it is warmer now, and the temperature is rising faster now, than at any time in the last one or even two thousand years. %26quot;
%26quot;Anecdotal evidence of wineries in England and Norse farmers in Greenland do not amount to a global assessment.%26quot;
%26quot;On its website, NOAA has a wide selection of proxy studies, accompanied by the data on which they are based. Specifically, they have this to say on the MWP:%26quot;
%26quot;The idea of a global or hemispheric %26quot;Medieval Warm Period%26quot; that was warmer than today, however, has turned out to be incorrect.%26quot;
%26quot;With regard to the %26quot;grapes used to grow in England%26quot; bit, here is some fairly solid evidence that grapes are in fact growing there now, denialist talking points aside.%26quot; http://www.reall.org/newsletter/v06/n08/...
Debunking of Wall St Journal claim in article
http://environment.newscientist.com/chan...
http://www.logicalscience.com/skeptic_ar...
http://gristmill.grist.org/skeptics
http://www.skepticalscience.com/
The Scientific Basis for Anthropogenic Climate Change
http://chriscolose.wordpress.com/2007/12...
The 2008 National Academy of Sciences Summary
Brochure on Climate Change
http://dels.nas.edu/dels/rpt_briefs/clim...
Scientific Opinion on Climate Change