Past and Present Climate
By Zack Guido | The University of Arizona | September 14, 2008
At night, illuminated satellites can be faintly seen moving across the sky. Some of them are imaging the Earth, recording data that is used to track weather and climate. On boats, thermometers thrown overboard measure temperatures of the ocean depths. From the tops of buildings, balloons float aloft to measure atmospheric conditions.
For the past 100 years, there has been a widespread scientific effort to monitor and understand the Earth’s climate system. This data has helped document recent climate change and has also enabled scientists to dive deeper into the geologic record to uncover past climatic conditions. Detailed climate records now exist for the last two million years. They combine with modern observations to reveal numerous long periods of glaciations, shorter-lived warmer times, regional temperature swings of 15 degrees F within a decade, and current warming that has made most regions warmer now than at any other time in the past 1,000 years.
A common catchphrase in geology is “knowledge of the present is the key to the past.” However, it is equally important to understand the past in order to clarify the future. Therefore, a comprehensive understanding of the climate system includes knowledge of:
Figure 1. Estimate of the Earth's annual and global mean energy balance.
| Enlarge This Figure |
Credit: Intergovernmental Panel on Climate Change, 2007
Climate records from deep oceanic sediments and wind-blown deposits span the last several million years, while gases trapped in ice-cores reveal the climate of the past 740,000 years. These and other records detail numerous glacial-interglacial cycles of the past three million years that are believed to be instigated by changes in solar energy striking the Earth's surface. These cycles are known as Milankovitch Cycles and they are characterized by global cool periods with major glaciations that recur about every 100,000 years, followed by shorter-lived warm periods. During these cycles, the minimum global average temperature has been about 9 to 11 degrees F cooler than the maximum temperature. Scientists believe changes in the Earth’s astronomical position slightly altered the radiative balance (Figure 1), which set in motion amplifying feedbacks and caused large temperature swings.2
Figure 2. As ice core records from Vostok, Antarctica show, the temperature near the South Pole has varied by more than 20º F during the past 350,000 years in a regular pattern that constitutes the ice age/interglacial cycles. Changes in carbon dioxide concentrations (blue) track closely with changes in temperature (red) during these cycles, but carbon dioxide levels are now higher than at any time during the past 650,000 years.
| Enlarge This Figure |
Credit: Marian Koshland Science Museum
During the glacial-interglacial Milankovitch cycles, temperature and atmospheric concentrations of carbon dioxide (CO2), a powerful greenhouse gas, recorded in the ice sheets of Antarctica have been nearly in lockstep (Figure 2). Since changes in past temperatures generally precede changes in CO2 by several centuries, many scientists believe that CO2 acted as an amplifier and not an instigator of temperature changes. The Intergovernmental Panel on Climate Change (IPCC) states that there is greater than a 90 percent probability that CO2 variations strongly amplified climate but did not trigger the end of glacial periods. For example, the IPCC states that Antarctic temperature started to rise several centuries before atmospheric CO2 during past glacial terminations (Figure 2).1
Measurements of atmospheric gases trapped in ice cores indicate that in the past 700,000 years, CO2 varied within a range of 180 to 300 parts per million (ppm) and concentrations of another important greenhouse gas, methane, varied within 320 to 790 parts per billion (ppb) over this period.1 In 2011, CO2 concentrations topped 390 ppm, while methane reached 1,775 ppb in 2005.3
Changes in the Earth’s orbit and concentrations of greenhouse gases principally drove the temperature oscillations that marked the glacial-interglacial cycles during the past three million years. But the transitions into and out of the warmer and colder periods were not always gradual. Records show numerous periods with dramatic swings in temperature, with some local changes as high as 30 degrees F, occurring in as little as a decade.4
Abrupt climate change occurs when the climate system crosses a threshold, triggering a rapid transition to a new climate state. A climate system with thresholds behaves similarly to a tipping bucket that is balanced over a pivot—the bucket remains upright until one too many water droplets topples it. The ample evidence in the geologic record for rapid climate change suggests that a gradual change, such as an increase in CO2 or the melting of polar ice, may cause a large and rapid jump in temperature.
In the geologic record, the most notable rapid climate change occurred approximately 12,800 years ago, marking the beginning of the 1,200-year cool period known as the Younger Dryas. Ice core analyses indicate the period began with cooling occurring in a few, decade-long intervals, whereas the end was marked by an increase in regional temperature of about 15 degrees F over 10 years.4 In the geologic records, warming is often more rapid than cooling.
Another intensely studied rapid climate change occurred around 8,200 years ago. Scientists hypothesize mammoth lakes impounded by the North American ice sheet were able to drain catastrophically into the north Atlantic Ocean during the retreat of the massive ice sheet. This temporarily halted oceanic circulation and caused cooling. Evidence suggests that temperatures around the north Atlantic Ocean fell by as many as 18 degrees F, while Europe may have cooled by about 4 degrees F.4
The rapid climate change of the Younger Dryas and the event around 8,200 years ago were not uncommon in the past. Scientists believe that more than 24 events of similar duration, degree of temperature change, and global extent as the Younger Dryas occurred in the last 110,000 years.4
The IPCC states global average surface temperatures rose by approximately 1.3 degrees F between 1906 and 2005, and the rate of warming over the last 50 years is almost double that over the last 100 years.5 The Arctic is experiencing an even greater temperature change, with increases of about 2.5 degrees F in the past 100 years.5 The IPCC also states precipitation has generally increased over land north of 30°N latitude between 1900 and 2005 but has displayed a downward trend in the tropics since the 1970s.5 Droughts have also become more common since the 1970s, especially in the tropics and subtropics.5
With respect to the Southwest, temperatures in Arizona and New Mexico have also been rising, particularly since the mid-1970s. Since 1976, the average annual temperature increased in Arizona and New Mexico by 2.5 and 1.8 degrees F respectively.
Figure 3. Atmospheric concentrations of carbon dioxide during the last several decades - the Mauna Loa or Keeling curve.
| Enlarge This Figure |
Credit: Marian Koshland Science Museum
Since the beginning of the Industrial Revolution in the late 1700s, the burning of fossil fuels, deforestation, and other human activities have contributed to rapid increases in atmospheric concentrations of greenhouse gases such as CO2 and methane. In the mid-eighteenth century, the estimated atmospheric concentration of CO2 was 280 ppm.6 By 2007, that number stood at approximately 383 ppm (Figure 3).7 Methane concentrations showed a similar meteoric rise, increasing from approximately 700 ppb at the beginning of the Industrial Revolution to about 1,775 ppb by 2005.3
At no other time in the last 700,000 years has atmospheric concentrations of CO2 or methane been as high as they are presently (Figure 2). In addition, available evidence indicates most regions are warmer now than at any other time since at least 900 AD.8
Using sophisticated global climate models (GCMs), the IPCC concluded that average global surface temperatures will likely rise by an additional 2.0 to 11.5 degrees F by 2100.9 The magnitude of the change will depend on which emissions scenario most closely approximates real-life emissions.
- Jansen, E., et al. 2007. Paleoclimate. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Alley, R.B., et al. 2003. Abrupt climate change. Science, 299: 2005-2010.
- Forster, P., et al. 2007. Changes in Atmospheric Constituents and in Radiative Forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Alley et al., 2002. Abrupt climate change: inevitable surprises. Washington, D.C.: National Academy Press.
- Trenberth, K.E., et al. 2007. Observations: surface and atmospheric climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Miller, K., C. Roesch, D.J. Stewart and D. Yates. 2006. Climate change and water resources: a primer for municipal water providers. AWWA Research Foundation, Denver, Colorado and University Corporation for Atmospheric Research (UCAR), Boulder, Colorado.
- Le Quéré, C., et al. 2008. Global carbon project: carbon budget and trends 2007. http://www.globalcarbonproject.org/carbontrends/index.htm (Last accessed April 17, 2009).
- National Academy of Sciences. 2008. Understanding and Responding to Climate Change: Highlights of National Academies Reports.Washington, D.C.: National Academy Press.
- Meehl, G.A., et al. 2007. Global climate projections. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.