Global warming is just one facet of human-induced global environmental change, and facet that has been shown to be occurring. Global warming is the phenomenon of mean (average) global temperature rising.
The Earth has experience global warming and global cooling numerous times in the 4+ billion years of Earth history, and will presumably continue to do so until the planet ceases to exist. The Earth (and multicellular llife) have survived both higher mean global temperatures and higher levels of CO2, and these extremes obviously occurred naturally due to factors such as Milankovich cycles, plate tectonics, extreme volcanic activity, etc. However, humans have not been around for billions of years. In fact, we have not been around for millions of years. The first remains of modern humans appear a mere 150,000 years ago. We are adapted for the kind of climate we have seen in the past 200,000 years, not a greenhouse world or a waterworld. The fact that the Ordovician, the Devonian, and the entire Mesozoic were extremely warm is irrelevant, because that was hundreds of millions of years ago.
Climate generally happens on the timescales of decades to centuries and longer. Weather happens on the timescale of hours to days to weeks to years. Climate influences weather, but weather may not directly reflect climate. So even though this winter has been extremely cold and severe in the US, it does not disprove global warming (it may, in fact, be evidence OF global climate change, but I will set that aside due to the relative complexity).
So, back to the CLIMATE issue, which is separate from day-to-day or season-to-season weather. We know the Earth is warming, and we can be reasonably sure it is due to greenhouse gases. Greenhouse gases are a perfectly normal and healthy part of the Earth's atmosphere. By effectively "holding in" some infrared radiation that our planet reflects off its surface, it helps keep the planet warm enough to sustain life.
In the Earth's past, we know that carbon dioxide levels and mean global temperature were extremely closely correlated (Figure 1: http://www.ncdc.noaa.gov/.../glob.../temperature-change.html ). Correlation does not imply causation, but we know from chemistry how the molecules of the greenhouse gases oscillate in such a way to promote the Earth to "hold in" heat, so the proposition that increases in carbon dioxide levels will cause global temperature increases is a perfectly plausible explanation.
We also know what how carbon dioxide levels have fluctuated over the past 800,000 years (Figure 2: http://cdiac.ornl.gov/images/air_bubbles_historical.jpg ). Notice how carbon dioxide did not exceed 300 ppm (parts per million) over the past 800,000 years. Now consider Figure 3 ( http://www.esrl.noaa.gov/.../ccgg/trends/co2_data_mlo.pdf ). While there are clearly seasonal variations in carbon dioxide levels, the trend is approaching 400 ppm! Humans, along with numerous animal species, have never encountered carbon dioxide levels so high!
Just as you would expect with rising concentrations of greenhouse gases, global temperatures are also rising. Figure 4 shows the change in mean global temperature as the deviation (the temperature anomaly) from the average temperature in the mid-twentieth century (a value we can assume is already elevated from what it naturally "ought" to be) ( http://data.giss.nasa.gov/gistemp/graphs_v3/Fig.A2.pdf ). Notice also that while there are fluctuations in mean global temperature, the general trend is rising. The rises in global temperature are not evenly distributed around the globe due to climate variables driven in part by the current positioning of the continents (Figure 5 : http://data.giss.nasa.gov/gistemp/graphs_v3/Fig.B.pdf ).
Some object that the mean temperature of Earth is not increasing as much as it "ought." But pull out your old chemistry book and think about what happens during phase changes. That's right: the temperature does not change during the phase change, effectively letting the melting process of arctic ice "eat up" some of that excess heat.
This melting of Arctic ice is one of the large reasons why global warming is so alarming. How many global cities are on low-lying coasts? How many island nations are there? How many nations with long coasts? The economic and cultural costs of sea-level rise alone are alarming. Our planet is only years away from the first climate refugees (and some scholars will tell you we already have climate refugees because of the way climate change creates more severe weather patterns).
So, in conclusion, global warming is real, and NOT just in name only. The globe is literally warming, and we know that increases in atmospheric carbon dioxide and other greenhouse gases are the reason why. But the biggest questions are a.) Are humans to cause?, and b.) Can we do anything about it? I am only qualified to answer the first.
From wood or coal in a stove to the coal, oil, and natural gas burned in industry and transportation (starting in full force in the late 18th century and continuing to present day), we know burning of any organic results in the release of greenhouse gases. We know that modern industry burns far more organics than did earlier civilizations,who used them primarily to cook, heat homes, and forge tools. We know, as explained above, that greenhouse gases cause global warming. Therefore, if human burning of fossil fuels is the cause of global warming, we would expect to see a massive increase in carbon dioxide and other greenhouse gases around the year 1800, correlated with the Industrial Revolution. This is what scientists call a hypothesis. It is both testable and falsifiable.
When you test this hypothesis, you find that the data supports human-induced global warming due to greenhouse gas emissions (Figure 6: http://www.globalchange.gov/HighResImages/1-Global-pg-14.jpg ). Note in Figure 6 that the exponential growth in greenhouse gas levels begins around 1800, exactly as our hypothesis predicted. Note also that our hypothesis is not purely deductive reasoning, so we cannot say with 100% certainty that humans are the source of all this carbon dioxide, only that it is very likely that we are. Could there be another cause for the increase in greenhouse gases around 1800? Climate models suggest "no" (Figure 7: http://www.globalchange.gov/HighResIm.../1-Global-pg-20L.jpg ).
For more info on Milankovich cycles: http://en.wikipedia.org/wiki/Milankovitch_cycles For more info on the climate history of Earth: http://nature.nps.gov/.../climate_change_earth_history.cfm
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