P.J. Rovero, Meteorologist and Oceanographer
Last updated: July 16, 2016.
Global climate change is a fact. The problem is that
the climate has always been changing, and not always due
to human activity. And during the peak of the last interglacial optimum, sea level was up to 6 meters higher, and the temperature up to 3 degrees Celsius warmer than at present. And somehow humans, penguins, and polar bears survived it all.
Many scientists and politicians like to focus on the very short 135 +/- year long period of "instrumented" meteorological records when discussing climate change. But the truth is, we have detailed scientific evidence for climate change going back hundreds of thousands of years.
The study of past climates, particularly those before historical instrumented records, is called paleoclimatology. Paleoclimatologists use a number of proxies to substitute for the lack of actual instrumented temperature measurements. These include:
The time resolution of the proxy data decreases as we go back in time. Some climate proxies have limited periods of record. For example, Carbon-14 dating is limited to only 60K years; ice core data go back 800K years, and oxygen isotope fractionation in sedimentary rocks goes back millions of years. Figure 1 shows paleotemperature proxies going back over 500 million years.
Figure 1. Rohde and Royer, et al. paleotemperature graphs composited, Source: http://en.wikipedia.org/wiki/File:All_palaeotemps.png
Dr. Rohde has prepared a whole series of graphics relating to past temperatures as well as other aspects of climate change at http://www.globalwarmingart.com.
Additional forcing of global climate changes comes from the precession of the earth's poles, the eccentricity and obliquity of the earth's orbit around the sun. In fact, the 100 Kyr, 41 Kyr, and 26 Kyr spectral components of these predictions are found in ice cores and sea floor sediment data. There are, however, significant phase shifts between the times of maximum and minimum insolation and the climate shifts.
Some of the most compelling data comes from the analysis of Antarctic and Greenland ice cores. The core series going back farthest in time are the Vostok (450 Kyr) and European Project for Ice Coring in Antarctica (EPICA , 800 Kyr). And what do these cores show? As illustrated in Figure 2 below, there have been a series of four long, cold glacial periods interrupted by relatively short interglacial periods over the last 420 Kyr. The EPICA core goes on to show four more glaciations in the 400 Kyr previous period. The Vostok and EPICA cores agree quite well over the common period of record. Greenland ice cores show evidence of repeated major temperature shifts (12 C, or 22 F) in as little as 50 years in the period 12 - 15 Kyr ago.
Figure 2. 420,000 years of CO2, CH4, Oxygen isotope ratios, insolation, and Temperature from the Vostok ice core. (Reference)
There are additional proxies for changes in solar irradiance. While the changes in total irradiance over an 11 or 22 year solar cycle are small (order 0.1%), changes in ultraviolet (UV) exceed 1%. Other indirect effects include modulation of the earth's exposure to cosmic radiation and the resulting cosmogenic generation of beryllium-10 (Figure 3), carbon-14, and chlorine-36. Increased cosmic radiation may also be associated with production of cloud condensation nuclei and changes in the overall earth albedo. A new model of solar activity (July, 2015) predicts a 60% decrease in solar activity (sunspots, flares, etc.) over the next few solar cycles.
Figure 3. Beryllium-10 concentration vs Sunspot Number
We live in the most recent interglacial period -- the same one in which agriculture developed, along with the rest of "recorded" human history. What the ice core record shows is increasing CO2, CH4, and temperature for the last 20,000 years. The Industrial Revolution accounts for only the last 1% of this time period -- a figurative "boil on the butt" of an existing, non-anthropogenic global warming trend. Temperatures haven't been increasing only for the last 200 years, they've been increasing for 20,000 years.
Figure 4. The rise of sea level since the last glacial maximum. (Reference)
About 125,000 years ago, during the last interglacial optimum (the Eemian period), sea levels were 4 to 6 meters higher than they are today, and Northern hemisphere temperatures up to 3 degrees Celsius warmer than today -- without significant anthropogenic land use changes and emissions of CO2, soot, or CH4. As shown in Figure 4, sea level was more than 120 meters below current levels at the time of maximum glaciation 22,000 years ago. The average rate of sea level rise over the last 20,000 years is greater than the rates observed over the last two centuries.
As more than one observer has noted, analysis of this time series might lead one to believe we are getting ready start another period of glaciation. It is unclear what triggers the periodic rapid warmings and coolings -- it is not only predicted orbital cycle changes in insolation, or changes in CO2/CH4 levels. Other parameters that may contribute include dust (from soil erosion and volcanic eruptions, which have been observed in the ice core and ocean sediment records) and small changes in the amount or character of solar or cosmic radiation. The consensus view appears to be "nobody knows for sure, but it probably wasn't human beings".
In the Antarctic, the temperature fluctuations lead the changes in CO2 and CH4 by several hundred years, when exactly the opposite might be expected by the most popular global warming theories. Both ice core and stomatal density/index data show this. Recent articles categorize this as an anomaly, but CO2 is well mixed in the atmosphere, with regional seasonal variations on the order of only 1% to 2%. And the explanations (more fossil fuel burning in the Northern Hemisphere) point to factors that probably were not significant before 2,000 years ago.
We are in a period of global warming that started 20,000 years ago. Human beings have contributed to detectable climate change on the micro-, local and regional scales through changes in land use and population (and energy consumption) concentration. The urban heat islands are a good example. Human beings may have contributed to changes in global climate through changes in land use and industrial emissions of CO2, CH4, and other "greenhouse" gases. The human population has increased dramatically during this period with both warming temperatures and increased CO2 - it is difficult to argue that CO2 has been deleterious to human existence to date. The best evidence shows that per capita CO2 emissions are increasing as living standards improve over most of the world. Combined with projected population increases to 11 billion or more, it does not bode well for decreases in either emissions or atmospheric CO2 levels. The certainty and magnitude of those potential anthropogenic changes is a matter of current scientific research and political dispute.
In view of the paleoclimatology record, I believe that some scientists have gone too far propagating a crisis mentality and supporting sweeping political "solutions" to a problem that may not even exist. Those scientist who fudge the data (and won't release it), and who have tried to exclude scientists with contradictory data from publication have certainly gone too far. Scientists who switch from the science to political advocacy risk damaging science.
In view of the uncertainties in the paleoclimatology record, I do not believe that sweeping political "solutions", particularly those that concentrate power in governments, are needed at this time. It is appropriate to promote energy conservation and renewable sources of energy. That can be done without the hype, global treaties, global targets, and so-called "cap and trade" tax increases.
If you believe in anthropogenic global warming, take direct action to reduce your energy and greenhouse gas (carbon, methane, CFC, etc.) footprints. For example, don't take two trips to Scandinavia in Air Force One (one for the Nobel Peace Prize, one for the "climate summit") when one trip will do. Get serious about reducing your energy use and greenhouse gas emissions. Stop opposing the efforts of others to build non-carbon-emitting solar, wind and nuclear energy projects. Lead, or get out of the way.
Reduce your energy use at home and work.
If you are not willing to do these yourself, stop trying to tax, legislate, or otherwise get other people to do them