By Renee Hannon – Re-Blogged From http://www.WattsUpWithThat.com
In the mid-1900’s many scientists were suggesting the Earth was cooling. Now scientists are forecasting global warming. Indeed, instrumental data shows global temperatures warmed by approximately 1-degree C during the past 165+ years. With warming rates of 0.5 to over 1.3 degrees C per century this has caused considerable alarm for many. This recent warming is commonly attributed to increasing greenhouse gases, primarily CO2.
This post examines natural paleoclimate trends and simple characteristics of past and present climate cycles at different time scales. Data suggests distinct differences between short-term climate variability and longer-term climate change. This is important because short-term climate variability can be misinterpreted as underlying climate change resulting in poor science and potentially worse policy decisions. This post compares modern instrumental trends to paleoclimate trends. This comparison reveals modern warming has characteristics of natural short-term climate variability and not long-term climate change.
Comparing modern instrumental measurements to long-term paleoclimate data is not a simple task. They are vastly different types of datasets. Ice core paleoclimate isotope data are indirect indications of temperature (proxies) over millions of years compared to instrumental temperature measurements with high resolution of hours, days and decades. However, paleoclimate data cannot be ignored or dismissed when trying to understand present-day temperature trends. Paleoclimate characteristics and trends provide the overarching framework and climate history to better understand centennial temperature fluctuations and potential future global temperature tipping points. Earth’s natural baseline of historical climate must be established prior to any attempt to assign potential human impacts.
“Weekly or daily weather patterns tell you nothing about longer-term climate change (and that goes for the warm days too). Climate is defined as the statistical properties of the atmosphere: averages, extremes, frequency of occurrence, deviations from normal, and so forth.” Shepherd.
Climate timeframes and relative scales observed in paleoclimate data are illustrated in Figure 1. The glacial cycle repeats approximately every 100,000 years and consists of an interglacial and glacial period. Cold glacial conditions predominate 70% of the time while warmer interglacial conditions occur about 30% of the 100,000 years. This entire cycle has repeated four times over the past 400,000 years. The glacial cycle and occurrence of interglacial warm periods are commonly accepted as being influenced by the Milankovitch astronomical processes.
Figure 1: The climate framework using EPICA Dome C ice core temperature proxies. a) a glacial cycle over 100,000 years with warm interglacial periods in red and the long glacial period in between. Termination events and onset of the interglacial period are labeled. GM refers to glacial maximum. b) zoom in of the interglacial period (MIS 5e). Each interglacial period consists of a warming, plateau, and cooling segment, medium term events such as climate optimums and intervening cool events, and the smallest detectable climate variations in black that last only hundreds of years.
The glacial and interglacial periods are very different climate cycles and are composed of different patterns and events. The interglacial framework and short-term events superimposed on all segments of the interglacial period are examined below.
Interglacial Periods Define the Long-Term Underlying Trends
There are approximately five interglacial periods ranging in duration from ten to thirty thousand years during the past 500,000 years. The marine isotope stage (MIS) terminology is used in this post. Figure 2 shows the correlation of four of the interglacial warm periods. MIS 7e was omitted from this correlation due to its unique and unusual character of several short interglacial periods. Further discussion of MIS 7e can be found here.
The entire cycle for the interglacial warm period is defined from the glacial maximum to the next significant low temperature minimum. This cycle is subdivided into warming onset, plateau, and cooling segments. Figure 2 shows the systematic and repeatable sequence of these three segments.
Figure 2: Correlation of interglacial warm periods over the past 400,000 years. Present day HadCrut data is shown in red on the Holocene MIS 1 temperature proxy curve. Three key segments are highlighted; red is the onset warming, yellow is the interglacial plateau, and blue is interglacial cooling. Multi-millennial events are labeled as optimum, Younger Dryas (YD), 8.2 kyr event, and corresponding intervening cool events in past interglacial periods. Dansgaard-Oeschger (D-O) events are labeled and are mostly associated with the glacial period. Note the high frequency temperature fluctuations superimposed on the various segments of all interglacial periods.
Amplitude and Duration: The most significant events are terminations of the glacial period and rapid onset of global warming of the interglacial period. These events are frequently referred to as Terminations I-V. The interglacial warming onset shows the largest temperature increases of 5-7 degrees C globally and up to 12 degrees C in the Antarctic dome C data. This dramatic increase in temperature occurs rapidly over 5,000 to 7,000 years as glacial sheets begin to decrease in size, sea levels rise and greenhouse gases increase. This warming process eventually reaches an interglacial optimum and plateau.
The interglacial plateau shows variations in temperatures of approximately 1-4 degrees C and lasts from 10,000 to greater than 20,000 years. MIS 7e plateau was an exception lasting only 6,000 years. Earth is currently within the Holocene interglacial plateau that has lasted for more than 11,000 years so far. During this time, sea level is greater than minus 20 meters relative to present day and the Northern Hemisphere (NH) is predominantly ice free except for the Greenland ice sheet (Berger et. al).
The interglacial plateau is followed by global cooling of 4-6 degrees C and up to 8 degrees C in the Antarctic data that takes 7,000 to 13,000 years to re-enter the next glacial period. Many scientists propose that decreasing obliquity, or Earth’s tilt, is responsible for initiating the cooling tipping point. The initiation and growth of ice sheets occurs, ocean temperatures begin to cool, sea level falls and greenhouse gases gradually decline during global cooling.
Rate of Change: The temperature rate of change was determined for the warming onset, plateau and cooling segments of the interglacial period. Trendlines are calculated using the linear regression analyses by the “least squares” method shown in Figure 3.
Figure 3: MIS 5e as an example for establishing trends and rate of change for the interglacial period segments. The warming and cooling trends have a strong correlation coefficient (R2), whereas the plateau tends to have a lower R2.
Figure 4 compares trendlines for each warming onset, plateau and cooling segment of the five interglacial periods. Note duration is in years. The starting points are pinned at zero, except for the global warming phase which was pinned at minus 10 degrees C. These simple trendlines provide visualization of the underlying long-term interglacial trends and removes medium and short-term internal variations as well as noise.
Figure 4: Rate of change or trendlines for the past five interglacial segments from EPICA Dome C temperature proxies. Initial starting points are pinned at zero except for the global warming trendline which is pinned at -10 degrees C. The length of the trendline approximates the duration of the interglacial segment. a) rate of change for the interglacial plateau segments. Marcott and May’s Holocene global reconstructions are included. b) rate of change for warm onsets also referred to as Terminations and c) rate of change for interglacial cooling segments.
Interglacial trends over the past 400,000 years exhibit steep warming onsets, slower cooling rates and nearly flat plateaus. Average warming onset rate of change is approximately 2.0 degrees C/millennium in the Antarctic with exceptionally strong correlation coefficients of 0.98. Average plateau rate of change is minus 0.01 degrees C/millennium (excludes MIS 7e) with weaker correlation coefficient of about 0.5. Interglacial cooling is less than 1.0 degree C/millennium with strong correlation coefficients of 0.95. Since the warming rate is twice as fast as the interglacial cooling rate, the typical interglacial period has an asymmetrical pattern