1. Natural Variability (pg. 384)
1. Geological Time Periods (Slide 1).
· X-axis = several hundred million years (going back to Paleozoic); Y-axis = temperature anomaly. Note that global average temperature was about 10 C warmer than present, except for two periods.
· Currently, we are emerging from an ice age, so expect temperatures to be increasing.
· Shorter Time Periods (Fig. 14.5)
· X-axis = about 20,000 years. Continued global warming, as expected when emerging from an ice age, with a couple of notable major climate shifts.
· “Younger Dryas” about 11,000 years ago, a cold spell
· “Holocene Maximum” (climatic optimum) about 5,000 years ago, a warm spell
· Historical Time Period (Figure 14.6)
· X-axis = about 1000 years. Northern hemisphere was about 0.2 C below “average” until early 1900’s when strong warming trend set in.
· Only since about 1900 have thermometers been widely used. Before then the temperature has to be inferred through scientific detective work, and is therefore imprecise (see points 1-8, pg. 386).
· Recent Past (Figure 14.7)
· X-axis = about 150 years. Global average was about 0.4 C below “average” until 1950’s.
· From 1950 to 1980 there was actually a period of global cooling, especially during the 1970’s.
· Onset of the industrial age (mid-1800’s) roughly coincides with onset of renewed warming.
2. Possible Causes for Natural Climate Variability (pg. 391)
· Changing Solar Energy Output
· We know from modern observations (Fig. 14.16) that solar output decreases with decreased sunspots so it is reasonable to infer that historical records of changing sunspot activity also mimic solar output changes (we don’t have long records of direct measurements of solar output). This idea is supported by Maunder minimum observation (pg. 401).
· Milankovitch Cycles (pg. 394)
· Eccentricity of Earth’s orbit around the Sun – 100,000 year cycle (Fig. 14.10).
· A highly eccentric orbit will take us far from the Sun and lead to cooling (at least part of the cycle).
· Precession (wobble) of Earth’s axis of rotation – 23,000 year cycle (Fig. 14.11).
· The wobble will eventually cause the N. hemisphere to point away from the Sun (i.e., experience winter) at the furthest point from the Sun (instead of the closest point to the Sun) thereby making the winters colder.
· Obliquity (angle) of Earth’s axis of rotation – 41,000 year cycle (Fig. 14.12).
· The less the obliquity the “weaker” the seasons (ranges from 22 degrees to 24.5 degrees).
· Changing Concentrations of Greenhouse Gases and Aerosols
· Water vapor (the most important greenhouse gas) and carbon dioxide probably changed in the past (see Fig. 14.13 for CO2 changes in Antarctica over past 400,000 years), thereby changing the natural greenhouse effect.
· These changes do not require anthropogenic effects, such as the industrial revolution.
· Unclear what came first – increasing CO2 or increasing T.
· Aerosols (small particles and liquid drops) can change global average temperature (see effect of Mt. Pinatubo eruption, Fig. 14.15). White aerosols reflect light (cool), while dark aerosols absorb light (warm).
3. Predicted Future Climate Variability
· Computer models take into account greenhouse gases, aerosols, solar variability, among other factors, and do reasonably good job of reproducing observed global temperature anomalies (Fig. 14.17).
· They predict about 3-4 C warming by 2100, depending on greenhouse gases, etc., which would take us about half way out of the ice age (see Slide 1).
· Intergovernmental Panel on Climate Change (IPCC) Report is the “gold standard”.