Global Warming And Climate Change Causes

In order to understand global warming, it's critical to understand its causes.  The first question many people ask is "if the climate changed naturally in the past, how do we know it's not changing naturally now?", or worse, they say "because the climate has changed naturally in the past, I'm sure the current global warming is natural."

Obviously it's true that there have been many, many natural climate changes in the past.  In order to determine if the current climate change is natural or influenced by humans, we need to know what caused those past climate changes.  So what was it?

Natural Climate Change Forcings

 
Two of the main causes of climate change on Earth - variations in the planet's orbital (Milankovitch) cycles, and variation in solar output.

Milankovitch Cycles: These are are the collective effect of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milankovic. The eccentricity, axial tilt, and precesson of the Earth's orbit vary in several patterns, resulting in 100,000-year ice age cycles over the last few million years.

Essentially as the planet moves and tilts closer to and further from the Sun, it can increase or decrease the Sun's impact on Earth, causing the planet to warm or cool, causing the climate to change.

Might the current climate change be due to changes in the Earth's orbital cycles?  The answer is no.  An often-cited 1980 study by Imbrie and Imbrie determined that

More recent work by Berger and Loutre suggests that the current stable climate may last another 50,000 years (unless something other than these cycles destabilizes it).  So while these cycles have caused the climate to change many times in the past, we're currently in a stable and long-term cooling portion of the Milankovitch cycles.

Total Solar Irradiance (TSI):  Also known as total incoming solar radiation (insolation).  The amount of solar radiation reaching the Earth can change as solar activity changes.  This is a known factor influencing global temperatures and thus climate.  Sometimes people will reference sunspots, which correlate fairly well with TSI (more sunspots generally means more incoming solar radiation), but solar irradiance is the specific factor impacting the Earth's climate.  Sunspots have no direct impact on global temperatures.

Since 1978 we've had satellites measuring TSI directly, and prior to that scientists use "proxies".  A proxy variable is something that is probably not in itself of any great interest, but from which a variable of interest can be obtained.  For example, climate scientists use tree rings and ice core layers as proxies to determine past global temperatures.  In the case of TSI, one such proxy is beryllium-10 concentrations, and another are the aforementioned sunspot numbers.

So the question again arises - could changes in TSI be responsible for the recent global warming?  Since we've had satellites measuring TSI directly since 1978, and this is the period of the greatest warming in recent history (0.5 degrees Celsius over the past 30 years), all we have to do is look at the satellite data to determine if solar irradiance has similarly increased over that period.

Again, the answer is no.  On average, TSI has remained essentially unchanged since 1978.  According to the Max Planck Institute for Solar System Research, it hasn't increased (on average) in about 70 years.

Similarly, sunspot numbers have remained steady over that period as well.

Additionally, no scientific study has concluded that more than one-third of the warming over the past 30 years could be from solar influences, and most attribute just 0-10% to solar effects.  About the theory that the Sun is directly responsible for the recent warming on Earth, geophysicist Ray Pierrehumbert at RealClimate put it best:

Who said scientists don't have a sense of humor?

Carbon Dioxide Feedbacks:  It's important to note that while most past climate changes weren't initiated by CO2, carbon dioxide did play a big role as a "feedback".  For example, because CO2 is less soluble in warmer water, as the planet warmed due to one of the natural causes discussed above, eventually the oceans reached a temperature at which they began to release more CO2 than they absorbed.  The CO2 then accumulated in the atmosphere, causing the planet to warm further (since it's a greenhouse gas), causing the oceans to release more CO2, etc. etc.

Generally in past climate warmings, it took the planet approximately 800 years to warm to the level at which this CO2 feedback kicked in.  This is often referred to as the '800 year lag'.  Sometimes this lag is mistakenly cited as evidence that carbon dioxide cannot cause global warming.  However, after the 800 year lag, CO2 amplified global warming for several thousand years.  While carbon dioxide did not initiate these previous warmings, it did amplify and cause further warming.  Plus we know that CO2 can cause warming, because that's the nature of a greenhouse gas.

So if we know these natural forcings aren't causing global warming, what are the other possible explanations?

Anthropogenic (man-made) Global Warming

The difference between the current and previous climate changes is that now there is a temperature-independent source of CO2 emissions - humans burning fossil fuels.  We know that atmospheric CO2 levels have increased 40% since the Industrial Revolution, we know that humans have caused almost this entire increase, and we know that increased levels of greenhouse gases cause the planet to warm.

Additionally, there are many basic scientific facts which can only be explained if the current global warming is being caused by an increased greenhouse effect.  For example, the planet is warming as much or more during the night than day (also known as decreasing diurnal temperature range [DTR]).  If the warming were due to the Sun, the planet should warm a lot more during the day when it's being bombarded with solar radiation.  Greenhouse gases absorb infrared radiation all the time, so they warm the planet regardless of time of day.  Another example is that the upper atmosphere including the stratosphere is cooling because the greenhouse gases trap the heat in the lower atmosphere.  If warming were due to the Sun, it would be warming all layers of the atmosphere.

For more examples, see the Fingerprints of Human-Caused Climate Change Wiki.

And perhaps most importantly, the radiative forcing from greenhouse gases and CO2 in particular has been by far the largest over the recent decades.  Radiative forcing is essentially the difference between the incoming and outgoing radiation energy on Earth.  The reason the planet is warming is that the incoming radiation is greater than the outgoing.  This imbalance is primarily due to increasing amounts of greenhouse gases in the atmosphere:

In short there is a lot of evidence supporting the theory that human greenhouse gas emissions are causing the planet to warm, including fundamental physics.

Are there any other possible explanations?

Galactic Cosmic Rays

There is one other possible theory, proposed by Henrik Svensmark.  The theory suggests that when the Sun's magnetic field increases, it blocks more galactic cosmic rays from striking the Earth, and theoretically cosmic rays might seed cloud formation.  In general, clouds cause the planet to cool by reflecting sunlight.  So the theory goes that an increased solar magnetic field blocks more galactic cosmic rays, causing fewer clouds to form, causing less solar radiation to be reflected, causing the planet to warm.

There are some fundamental flaws with this theory.  For example:

• There is little evidence from cosmic ray monitors on Earth that there has been any long-term trend (increasing or decreasing) in galactic cosmic ray flux reaching the planet.  If the cosmic ray flux is not decreasing, the theory doesn't work.
• There is little evidence from satellite monitoring of any long-term trend in low cloud cover.
• Unlike the anthropogenic global warming theory, the galactic cosmic ray theory cannot easily explain the cooling of the upper atmosphere, greater warming at night, or greater warming at higher latitudes.
• There is little physical evidence that galactic cosmic rays successfully seed clouds in reality.  As a 2006 study concluded, "the variation of ionization by galactic cosmic rays over the decadal solar cycle does not entail a response...that would explain observed variations in global cloud cover...the variation in radiative forcing resulting from a response of clouds to the change in galactic cosmic ray ionization...is smaller than the concurrent variation of total solar irradiance."
• Another recent study concluded that "cosmic ray variability could at the very most explain only a quarter of the changes in cloudiness."
• Another recent study concluded that this theory isn't plausible because solar activity - including magnetic field - has not increased over the past 30 years.
• Another recent study concluded "no response to variations in cosmic rays...was found in marine low clouds in remote regions."
• Calogovic et al. in 2010 find "no response of global cloud cover to Forbush decreases at any altitude and latitude." In other words, yet another study finding galactic cosmic rays don't successfully seed clouds.
• Kulmala et al. in 2010 also find "galactic cosmic rays appear to play a minor role for atmospheric aerosol formation events, and so for the connected aerosol-climate effects as well."

In short, there is little scientific evidence supporting the galactic cosmic ray theory, and an abundance of evidence undermining it.  While the theory has not yet been completely disproven, it is highly unlikely to be able to explain any significant fraction of the recent warming of Earth.

Natural Oscillations

Some people have mistakenly tried to blame the recent global warming on natural oscillations like the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Atlantic Multidecadal Oscillation (AMO), and so on.  In a recent paper, a few Australian 'skeptical' scientists suggested this possibility, even though their research did not support this conclusion.

There are two physical reasons why these oscillations cannot cause long-term warming (or cooling) trends:

1) They're oscillations!  This means they alternate between positive (warm) and negative (cool) states, which cancel eachother out over the long-term, as you can see here:


While these oscillations have a large impact on short-term year-to-year global temperatures, they don't impact long-term temperatures.  These kinds of short-term impacts are why climate scientists generally look at trends over periods of 30 or more years, and why some people come to incorrect conclusions (i.e. "global warming has stopped") by looking at just a few years of data.

2) These oscillations have the capacity neither to create nor retain heat.  All they do is move it around between the oceans and air.  So we know they're not causing the current global warming, because both the oceans and air are warming.
 

To summarize, there is a preponderance of evidence supporting the theory that human greenhouse gas emissions from burning fossil fuels is primarily responsible (approximately 80-90%) for the warming of the Earth over the past 30 years, and for at least 50% of the warming over the past century (though a significant amount of the warming in the first half of the 20th century was due to increased solar irradiance).  There also appears to be no other plausible theory which can explain any significant amount of the warming over this period.  Physics strongly supports the anthropogenic global warming theory.