Another Good Summary by Professor Scott Mandia, SUNY
Human beings evolved and established their cities, their agricultural zones, and their culture during the past ~10,000 years - a time of great stability in global climate - the Holocene geological epoch. Temperatures and CO2 levels changed very little during the pre-industrial, when humanity was a small fraction of the today's population, when man and his livestock composed only a few percent or less of all vertebrate biomass and wild animals still dominated life on land. The discovery and consumption of millions of years of fossilized concentrated photosynthetic energy all within a few centuries, in the form of oil, methane, and coal, has allowed man to rapidly dominate the planet. CO2 levels have gone up 43% since pre-carbon era times. Now, at the turn of the 21st century, man and his livestock comprise 97% of all vertebrate biomass, and all vertebrate wild land animals are just the remaining 3%. If you think it's suspicious that YOU should find yourself alive in this one rare moment in geologic time when this domination happened, don't be. So rapidly has our population multiplied, that half of all people who ever lived are alive today. The odds of YOU being alive now are rougly 50/50, so no need for surprise.
CO2 is a greenhouse gas - it allows short wavelength light from the hot sun to easily pass through it, heating the ground. However, it has strong absorption bands in the far infrared, where the Earth itself re-radiates its warmth back out into space. There is no uncertainty in this simple fact, discovered over 100 years ago by physicists. Raising CO2 levels in the atmosphere is like inserting a layer of fiberglass insulation between the ground and outer space, such that the "R-value" of the atmosphere in the infrared goes up. It's like throwing a blanket over the sun-warmed ground and then asking how well the ground can now cool off. As common experience shows, when you raise the thermal resistance, the temperature difference between outer space and the ground must go up in order to radiate away the (constant) energy input from the sun. CO2 and water vapor at the two strongest greenhouse gases in the atmosphere. 99% of our atmosphere is N2, O2, and Ar, and none of these are greenhouse gases. The sun-warmed ground and the air above it MUST warm up if we raise CO2 levels by 43%. This is simple basic physics, well codified in the "heat equation", and about which there is no uncertainty. And, it is observed directly with AERI spectra, showing increased radiative forcing of 0.20 watts/square meter per decade corresponding to the 22 ppm of increased CO2 between 2000 and 2010 (Feldman et al. 2015), in nagreement with theory.
How do we know this additional CO2 is due to us? There are several independent lines of evidence...
1. We know the CO2 is ours because of the sharply falling carbon isotope ratio C13/C12. Plants prefer C12 in doing photosynthesis. Fossil fuels were made by ancient plants and are enriched in C12 relative to the pre-industrial atmosphere. Pre-industrial atmospheric C13 reflects cosmic abundances generated by stellar nuclear fusion, and C13 is about 1.1% of all carbon atoms. Adding atmopheric carbon from fossil fuel burning therefore will decrease the C13/C12 ratio in a predictable way and the measured fall in C13/C12 over the past millenium agrees with that predicted from human sources (Francey et al. 1999). The steep atmospheric CO2 rise is due to fossil fuel burning and some due to the massive tree cutting since pre-industrial times as well, both preferentially adding C12 to the atmosphere, thus lowering the C13/C12 ratio in the observed way. We can also look at satellite maps of CO2 concentration and see they originate at centers of human population and industrial activity.
2. Human Carbon Production Rates Are Actually HIGHER than Atmospheric Carbon Rise Rate. Economists and global industry keep track of fossil fuel sales, power plant (mostly coal) fuel use, auto production, alternative energy use, etc. and have done so since the dawn of the fossil fuel age. The simple chemistry of carbon burning tells us how much CO2 is produced per kilogram of fossil fuel burned. This all goes initially into the atmosphere, except a tiny fraction which is scrubbed out and sequestered in a few power plants. There is little uncertainty in these facts. Sharp as it is, the rate at which atmospheric CO2 is rising is only 55% the rate at which we are pumping it into the atmosphere. The other 45% is continually dissolved into the ocean (~27%, measured to have the same C13/C12 ratio as the atmosphere) or taken up by land (~18%), mostly by photosynthesis).
3. This CO2 Cannot be from Volcanoes. The additional CO2 cannot be due to volcanoes because the measured volcanic emissions C13/C12 ratio matches that of the pre-industrial atmosphere, not that of fossil fuels. Also, the observed century long average of volcanic emissions of all isotopes of CO2 is less than 1% of the rise rate in the atmosphere.
How Do We Know Global Temperatures Rises Are Real?
The "Urban Heat Island" claim has been thoroughly debunked. This effect was well-understood from the start, and even without calibrating it out, there is virtually no contribution to global temperature records due to this effect. There are independent measurements showing the Earth is warming - the rapid drop in Arctic Ocean permanent ice, the melting of the Greenland and Antarctic ice sheets from gravity measurements, the polar migration of plant hardiness zones, the migration of animals, the melting of continental glaciers measured by their length and thickness, the earlier arrival of Spring as measured by flowering plants, the northward migration of tropical diseases, the rise rate of record high temperatures both day and night, and other indicators.
How Do We Know Global Temperature Rises Are Due to CO2?
First, a unique signature of heating caused by the greenhouse effect is a warming troposphere, but at the same time a cooling stratosphere (the higher CO2 in the stratosphere acts as a coolant because molecular collisions excite CO2 which de-excites by emitting IR radiation, thus damping (cooling) the molecular motion. And, it receives less outgoing IR from below because there is much more CO2 in the lower atmosphere). Second, another signature of greenhouse caused warming is for night-time temperatures to rise faster than daytime temperatures. This is also what we observe. Ground warmed by the sun in the afternoon tries to radiate to the night sky and the greenhouse effect inhibits this. Daytime temperatures rise too, but not as much because the solar heating takes time to heat the ground before the ground emits more IR radiation. The delay in heating shifts the radiating into the evening, when the blanket effect of CO2 takes effect.
Let's assume we double CO2 atmospheric levels from the 280 ppm of pre-industrial times up to 560 ppm, and then leave all other effects unchanged, and let the Earth adjust until incoming solar energy is once again matched by Earth thermal radiation to outer space, a process which takes of order a century. The resulting global average surface temperature change is defined as the equilibrium climate sensitivity. CO2 physics is well understood, first measured accurately in the 1950's by the U.S. military in order to understand the design of heat-seaking missiles. Doubling CO2 would, alone, raise the global average surface temperature +1.2 C. This is a straightforward calculation with no significant uncertainties. Now look at the additive positive feedback effects this temperature rise would cause...
Positive Feedbacks Making CO2-induced Global Warming Stronger...
1. Hotter air can hold more water vapor. Hotter air means faster moving molecules, and just as in juggling tennis balls, throwing them faster allows a juggler to keep more in the air at once, hotter air will keep more water molecules in the air without having them stick together and condense out as rain. It's a steep relationship - the air can hold fully 7% more water vapor by mass for each degree Celsius temperature rise. Water vapor is also a greenhouse gas (in fact, all asymmetric molecules are strong greenhouse gases - symmetric diatomic molecules are not - they have no electric dipole moment. Standard graduate level atomic physics). The absolute humidity of the air, averaged around the world, is such that there is more water vapor than CO2, and water vapor has about twice the greenhouse effect as CO2 alone. This is simple physics well measured and with no uncertainty. This additional humidy-induced greenhouse effect is a strong positive feedback to CO2 initiated warming. And, this rising absolute humidity is clear in the real data
2. Earth albedo (reflectivity to sunlight) is, and will continue to, drop at an accelerating rate. Rising temperatures are causing ice to melt, particularly the Arctic Ocean. It has taken all of the 20th century to melt through nearly all of the permanent ice. Within a few years, all permanent Arctic Ocean ice is expected to be gone. Seasonal ice is thin and melts quickly in early summer. This means that a significant part of the summer will have dark exposed ocean absorbing sunlight that would normally be reflected. This effect was not included in the IPCC studies of previous years. Note too that whether the Arctic Ocean ice is 10 foot thick permanent ice, or inches thin seasonal ice - they are about equally reflective and therefore have had little effect on Arctic temperatures. That period is now ending, and dark, absorptive open water rapidly comes to dominate the summer Arctic Ocean today and in coming years. So one cannot simply linearly extrapolate the past century's climate trend into the future - it will, in fact, be worse. This feedback is strongly positive and is beginning to be felt only in the past decade or so. This ice-free state is happening far faster than the IPCC AR4 predicted just a few years ago. Also, Greenland ice is getting darker, both from deposited ash from increasing western wildfires, and also from changes in the geometry of ice crystals after they partially melt from warmer temperatures.
3. Methane. Pound for pound, methane is 20-200x more powerful than CO2 depending on time scale from very short term, up to a century. However, it oxidizes into CO2 and H2O with a half-life of about 9 years. The effective greenhouse effect of methane depends on the time scale considered; over many decades, the factor is of order 20x. Methane releases from the Arctic permafrost are only now beginning to be felt. Up until now, methane atmospheric concentration has doubled mostly due to the methane produced by our livestock. Livestock numbers are fairly well known, as is the methane production that results. Methane from the melting Arctic is predicted to strongly add to greenhouse warming when temperatures have risen 2 C above preindustrial levels. We are halfway there already. Arctic methane is already being released today, at an accelerating rate. This is a strong positive feedback; Arctic methane alone is predicted to add 30% to the CO2 greenhouse forcing from CO2. It may be worse yet - as the magnitude of East Siberian permafrost methane release at this early stage of global warming is far higher than expected.
4. Cloud cover changes. This is the greatest source of uncertainty for predicting future climate when warming has gotten much stronger than today. The data is so far not good enough to say more than that cloud feedbacks are very likely positive. Theoretically, we would expect cloud feedbacks to be positive. Low clouds close to the ground are made of water droplets, are warmer at their tops, and dense and very reflective and hence they cool climate. High altitude clouds are colder at their tops, emit less radiation, reflect back downward the outgoing IR more strongly, are sparse and much more transparent to incoming sunlight, and act to warm climate. Observations and physics (Sherwood 2013) are indicating that climate-cooling low clouds will tend to dissipate in favor of climate-warming high clouds as the Earth warms. This is because the hotter surface enhances convection, mixing out the fog and low stratus by dehydration. Also, hotter oceans generate more powerful atmospheric convective storms which generate cloud tops all the way to the bottom of the stratosophere, where cirrus clouds form, albeit with significant remaining uncertainty when temperatures get much warmer than today's..
5. Aerosol changes. Volcanic aerosols tend to cool climate but last only a couple of years after the volcanic event, and are unpredictable and show no net change over long time scales. Human-caused aerosols are mostly from coal-burning, producing sulfate-rich droplets which are reflective and tend to cool climate. Aerosols can warm climate if they are dark (soot) and absorb sunlight. Aerosols can also nucleate cloud droplets which, at low altitudes, tend to cool climate. Measured aerosol effects are fairly strongly on the cooling side. What they do in the future depends strongly on unknown human actions. Despite the cooling effect of human aerosols (mostly now from rising Asian power plant coal burning), global temperatures have more than compensated and have been rising at a rapid rate. If we phase out coal burning or otherwise reduce aerosol pollution from fossil fuel burning, we may see more ground-warming sunlight raising surface temperatures. Hansen et al. (2005) quantified the forcing contribution of aerosols, and it is the second-most powerful effect on climate - a cooling effect, but less than have the magnitude of the warming effect of greenhouse gases. If China and other Asian countries ever clean up their air, the effect will be strong global warming, and it will be as immediate as more sunlight reaching the ground.
Summary on Feedbacks
CO2 levels have risen 43% above pre-industrial. Global temperatures have risen 0.9C. If we simply extrapolate this using climate models without methane or albedo climate feedbacks, and assume the next 40% rise (taking us up to a doubling; 1.4x1.4=2) is like the first 40%, you get a further 0.9C and a net 1.8C temperature rise (not equilibirum, note. The physical/thermal time scale for the atmosphere/Earth system is of order a century for some sort of equilibium to be reached). But from the feedbacks above the calculated true ECS is about +3 C. This also agrees with paleoclimate data at geologic times recent enough that continent placement and ocean currents are expected to be little changed. Several paleoclimate studies indicate ECS is 2-4 C, with 3C remaining the best estimate. We compare that with the observed +0.9 C with the 42% rise in CO2 and see this is entirely consistent with human causation. Climate models show it is not possible to account for the observed temperature rise when only natural variation is included (Meehl et al. 2004, and later). Realize that the largest physics uncertainty going into the future is our ability to model cloud feedbacks at higher temperatures - this does not affect observed cloud data in the past which is what is relevant for accounting for the observed climate today. Finally, note that even if, somehow against all physical reasoning so far, feedbacks in the more extreme climate of the future switched over to being negative feedbacks, this would only lessen the rate of further warming expected, not reverse it. It would change only the positiveness of the slope of the temperature change, not it's sign. That's straightforward math. Don't expect cloud feedbacks to save us.
Climate denialists will have you believe that uncertainties mean we should not sacrifice present lifestyles. But the greatest uncertainties in future climate modelling are in what humans will do about our aerosol pollutions and greenhouse gas emissions. Their reasoning is self-contradictory and goes like this... let's do little or nothing because we're not totally sure how bad things will get. By doing nothing, of course, we remove the largest unknown - what we'll do - and therefore insure that things will get much worse. This is Alice in Wonderland insanity.
Couldn't the Temperature Rise be Mostly Natural Variation, which May Not Last?
No. First, "Natural Variation" is not causeless. Physics is causitive. What else could be causing observed global warming? We have 3 solar cycles worth of satellite data which show convincingly that the sun's luminosity has shown a slight DEcrease over the past 60 years, especially in the past 30 years. The Pacific Decadal Oscillation has been in a cooling phase during the past 20 years, and overall is an oscillation, not a secular rise. The ocean cannot simply heat up continually without a source for that heat - and that is the higher air temperature right above it. The Atlantic MultiDecadal Oscillation is simply a name given to the temperatures in the Atlantic which show a temperature variation over the 20th century, including a slow rise. But the source of that secular rise is most naturally explained as the CO2 greenhouse warming of the atmosphere above it. There is no way for a net heating to the entire ocean to happen with no cause and source for that heat, again. There is no other natural climate variable identified which could have reversed the very slow cooling the Earth was undergoing during most of the last millenium (caused by the Milankovitch cycles, which are many thousands of years in oscillation). Natural causes show essentially zero contribution to global warming over the past 60 years. The ocean heat/cool cycles of the Pacific Decadal and El Nino/Southern Oscillation affect global temperatures (according to long term data) much less than the anthropogenic warming seen, and also happen over shorter time scales. Brief periods such as the past decade when global temperature rise is at a slightly slower rate correspond to times when the uneven transfer of heat from the surface ocean to the deeper ocean is happening more rapidly. Correcting global temperature rise for known and understood effects due to volcanic aerosol cooling, ENSO and ocean heat transfer variability, and the solar cycle show there has been no drop in the rate of global warming during the 21st century (Foster and Rahmstorff 2011, and condensed and explained here)
There it is, in a nutshell. Climate denial blogs and YouTubes you may run across will cherry pick small data intervals and deliberately leave out crucial information in trying to sound very "science-y" and bamboozle the reader into thinking they - the denialists - are smarter than all those unscrupulous scientists. Don't fall for it. Look at the real science, published in refereed science journal papers. Look at the real motivations. And "Follow the money" (as Watergate's "Deep Throat" admonished 40 years ago).
The truth is unsettling, and frightening. But ignoring it will be far worse.
