This will be quite long. I am not putting forth this much effort to help people resolve misconceptions concerning climate change only to have my thread flooded with troll/joke responses. If you wish to post such a response, please just PM it to me or keep it to yourselves.  Or make a new thread for such responses.  I would like to keep the discussion in this thread productive and civilized.

In case you missed it, here is a link to my last topic clearing up the misconception that global warming has halted in the last decade:

http://www.cougarboard.com/board/message.html?id=11820443

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With that out of the way, we can dive right in. One thing that I think we all can agree on is that climate has changed in the past. Many skeptics will even agree that it is changing now. The point that people get hung up on is the cause of recent warming. Luckily, by looking into past climate change, we can uncover secrets in decoding the current trends. In order to fully grasp this issue, there are a few concepts that we need to understand.

The first is the heat budget of the earth. In a nutshell, when the climate loses heat, global temperatures fall. Conversely, when the climate gains heat, global temperatures rise.  No internal mechanisms can create long term warming/cooling trends, there must be an imbalance in the earth's heat budget.  This follows directly from the laws of thermodynamics. 

Next, we must understand what is referred to as climate sensitivity, which is essentially a measure of how much global temperatures change from increased CO2 levels. The most common benchmark that I have seen is the temperature effect of doubling pre-industrial atmospheric CO2 levels.

We turn to the empirical evidence from past climate change to determine the earth's climate sensitivity, using measurements of global temperature and CO2 levels.  The following graph summarizes these findings (source: http://www.iac.ethz.ch/people/knuttir/papers/knutti08natgeo.pdf)

Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thick coloured bars indicate likely value (more than 66% probability). The thin coloured bars indicate most likely values (more than 90% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively.  (caption from http://skepticalscience.com/climate-change-little-ice-age-medieval-warm-period-intermediate.htm)

Notice the common theme of a climite sensitivity of about 3 C.  A second set of studies came up with a similar value of 3 C. (source: http://www.nature.com/nature/journal/v491/n7426/full/nature11574.html)

We have now seen the effect that atmospheric CO2 has on global temperatures.  Past climate change is evidence for high climate sensitivity, showing that human emissions can have a significant effect.  The number of studies over such a wide range of time periods, using different methods, studying different aspects of the climate average out effects from other forcings.

One might be tempted to think that this is still part of some natural cycle.  The word "cycle" suggests that things swing back for the heck of it, when that is not the case.  The climate doesn't change for no reason, it changes in reaction to various forcings on it, like solar output, volcanic activity, and greenhouse gas levels, all of which can be measured and correlated.  By studying the past, we can determine the effects of natural forcings on the climate.

To illustrate the effects of some of these forcings, I present two case studies, the Midieval Warm Period and the Little Ice Age. 

Midieval Warm Period

First, there is the misconception that the Midieval Warm Period was all around warmer than our climate today.  This is false.  While temperatures in certain regions, particularly the North Atlantic, were comparable to today's temperatures, there were also colder regions, all averaging out to early-mid 1900's levels.  (source: http://books.nap.edu/openbook.php?record_id=11676&page=R1)

Now we can move on to the known causes of the Medieval Warm Period.  This period occured during a period of high solar activity and low volcanic activity, both of which are well established positive forcings on the climate.  (source: http://www.sciencemag.org/content/289/5477/270.short)  These forcings stand in stark contrast to those we see today, so the warming cannot be attributed to the same causes.

Little Ice Age

One might wonder if whatever forcings brought us out of the Little Ice Age could be behind the current warming.  When we examine the causes behind the Little Ice Age, we find decreased solar activity and increased volcanic activity.  (sources: http://link.springer.com/article/10.1007%2Fs00382-010-0795-1, http://www.geos.ed.ac.uk/homes/tcrowley/crowley_science2000.pdf) Notice the contrast to the forcings behind the Midieval Warm Period.  This graph shows the solar output of the last 1000 years:

In recent history solar output and global temperatures are going in opposite directions, unlike the Little Ice Age.  The question is, why? 

More food for thought concerning the Midieval Warm Period and Little Ice Age:

http://www.nature.com/ngeo/journal/v6/n5/abs/ngeo1797.html

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We have now seen the effect that CO2 levels have on global temperatures.  We have also seen how we learn the effects of different natural forcings from past changes in climate.  It now only remains to show how the current warming cannot be explained by these natural causes.  Luckily, the climate is covered in "fingerprints" left by human emissions.  I will go through a number of them.

First, evidence that it is humans, not some natural cause, behind increasing CO2 levels:

• We emit about 30 billion tons of CO2 into the atmosphere annually. 
• CO2 from fossil fuels contains different ratios of carbon isotopes than would occur otherwise (source: http://www.esrl.noaa.gov/gmd/icdc7/proceedings/abstracts/keeling.rFF328Oral.pdf)
• Oxygen levels are falling in proportion to rising CO2 levels. (source: same as previous bullet) If CO2 were escaping from the ocean or other natural stockpiles, we would not see the oxygen decrease.
• Coral records show a drastic increase in carbon from fossil fuels, by examing the same ratios of isotopes. (source: http://www.sciencemag.org/content/309/5744/2204.abstract)

Now for evidence that this increasing amount of atmospheric CO2, caused by humans, is trapping heat (and thus causing warming as explained earlier).  Remember that more heat equals higher global temperatures.

• As measured by satellites, there is less heat escaping into space at the exact wavelengths that CO2 absorbs.(source: http://www.nature.com/nature/journal/v410/n6826/abs/410355a0.html)
• More heat is being redirected back down to the surface. (source: http://onlinelibrary.wiley.com/doi/10.1029/2009JD011800/abstract)

The following indicators list what would be expected specifically from greenhouse warming, but not warming from other forcings.  I then provide evidence that this is indeed occuring.

• Nights should warm faster than days.  This is exactly what we observe: http://www.knmi.nl/publications/showAbstract.php?id=706
• If the last century's warming could be explained by solar activity, then the stratosphere would be heating as well.  If, however, warming were caused by the greenhouse effect, the stratosphere would cool as less heat escapes from the lower atmosphere.  This is exactly what satellite measurements are showing: http://onlinelibrary.wiley.com/doi/10.1029/2002GL016377/abstract
• With a cooling stratosphere and warming lower atmosphere, the boundary between them (tropopause) should rise.  This also has been shown: http://www.math.nyu.edu/~gerber/pages/documents/santer_etal-science-2003.pdf
• The ionosphere (above the statosphere) should shrink and cool.  This also has been observed by satellites: http://www.ufa.cas.cz/html/climaero/topics/global_change_science.pdf

These fingerprints all point specifically to increased greenhouse gas warming from human CO2 emissions.

Now let's turn to some studies that analyzed the contribution of human versus natural influences on the climate in the last 50 years.  The following graph is a good summary of the literature.  As you can see, they all agree that there is a very strong human contribution to current warming trends. It also shows that natural forcings are negligible or slightly in the opposite direction.

sources:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2000ESASP.463..201T&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282004%29017%3C3721%3ACONAAF%3E2.0.CO%3B2

http://journals.ametsoc.org/doi/abs/10.1175/JCLI3966.1

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.167.2337&rep=rep1&type=pdf

http://thingsbreak.files.wordpress.com/2011/12/anthropogenic-and-natural-warming-inferred-from-changes-in-earths-energy-balance.pdf

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We have now established the effect that CO2 levels have on heat flux, and thus global temperatures. We have also seen the effect of natural causes like solar and volcanic activity by examining past climate change.  This allows us to accurately model climate trends.  The recent warming cannot be explained by the same natural forcings that caused past warmings.  Our climate is also covered with fingerprints of increased human CO2 emissions and the specific kinds of warming such greenhouse gases cause.  Everything points to one thing, that mankind and its CO2 emissions are mainly responsible for current global warming. 

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That about sums it up.  I would like to reiterate that I am in no way claiming that natural factors cannot influence the climate.  Earth's diverse climate history shows that this happens, and is the key to understanding climate trends, both past and present.  However, from this we see that current warming cannot be explained by the same forcings that drove previous warmings.  The following three graphs (taken from http://skepticalscience.com/coming-out-of-little-ice-age-intermediate.htm) show that natural forcings cannot explain recent warming.  However, anthropogenic causes alone also don't quite give the whole picture.