Estimating Historic Temperatures

Since the last glacial minimum, around 20,000 years ago, both CO2 concentrations and temperatures having been rising. There are various means of estimating temperatures for human history before modern temperature recording. These include ice cores covering the last 3000 years, the annual growth rate of tree rings and coral rings, and analysis of marine sediments.

Climate skeptics like to claim that global warming is not man-made because they argue that the world was warmer during the medieval period than it is now. In doing so, they make three critical errors:

• Their evidence does not show that the world was warm in the medieval period. It shows only that the North Atlantic was warm. It is well known that the North Atlantic is kept above what would be normal for its high latitude by the warm waters of the Gulf Stream. As this stream varies in strength, so does the temperature of the North Atlantic.
• The medieval climate was not changing rapidly. It was close to a state of equilibrium. Our climate is currently changing rapidly, and around half of the temperature rise due to anthropogenic CO2 is still to be felt in the coming decades.
• Skeptics overstate the medieval warming. A new temperature reconstruction has been published in the Swedish journal Geografiska Annaler: Series A, Physical Geography. The reconstruction covers the past 2000 years for the middle and high latitudes of the Northern Hemisphere. The results are shown below in Figure 1:

  

  Figure 1. Extratropical (30-90 N) northern hemisphere temperature reconstruction by Ljungqvist (2010). Northern hemisphere instrumental temperature records shown for comparison (CRUTEM land only, and HADCRUT land/ocean). Ljungqvist notes that this reconstruction shows a Roman Warm Period prior to AD 300, followed by a Dark Ages Cold Period (AD 300 to 800), a Medieval Warm Period (AD 800 to 1300), the Little Ice Age (AD 1300 to 1900), and modern warming in the 20th century. While there has been debate about how "globally consistent" these various warm and cold periods have been, they have long been recognized as prominent features of the extratropical Northern Hemisphere temperature record.

Instrumental Records

This section is under construction.

Recent Decades

The above graph shows Raobcore measurements for nearly 3 decades from 1979. Raobcore measurements are balloon readings. How accurate are they? They started in 1958, twenty years before satellite temperature records (which are renowned for their accuracy). Put the two methods side-by-side, and they tie together neatly, telling us that both of them are accurate, reliable tools. Raobcore and satellite measurements both indicate a rising trend in global average temperature of 0.2 degrees per decade.

The decade 2000 to 2009 was warmer than the decade 1990 to 1999, which in turn was warmer than the decade 1980 to 1989.

Adjusting for the Solar Cycle

Those Canberra politicians who have questioned whether global warming has continued over the last decade, including members of the coalition and Senator Fielding, have ignored, or perhaps been unaware of, the effect on temperatures of the solar cycle. Averaging 11 years in length, the sun oscillates between periods of higher output, accompanied by a maximum number of sunspots, and periods of lower output, accompanied by a minimum number of sunspots.

You can see form Figure 3 below that 1998, the hottest calendar year to date, was not only the year of the strongest El Nino on record, but also approaching a solar maximum. Both events contributed to abnormally high temperatures that year, but that does nothing alter the trend line, which is still rising.

The solar cycle means that warming does not increase smoothly from year to year, but rather tends to surge during solar maxima, and slow somewhat during solar minima. Therefore the solar cycle over the last decade has tended to mask the effect of increasing greenhouse gas emissions. The solar cycle will turn soon and by 2015 is likely to return to a solar maximum. When that occurs, global warming will be accentuated rather than masked. The middle years of this decade promise to be real scorchers!

Figure 3: Annual global temperature change (thin light red) with 11 year moving average of temperature (thick dark red). Temperature from NASA GISS. Annual Total Solar Irradiance (thin light blue) with 11 year moving average of TSI (thick dark blue). TSI from 1880 to 1978 from Solanki. TSI from 1979 to 2009 from PMOD.

Hottest on Record!

Although 1998 was the hottest calendar year (Jan - Dec) to date, the Earth has no interest in our calendar. The hottest year is actually the hottest period of 12 consecutive months. It turns out that record was set recently. The period May 2009 to April 2010 was hottest ever. The second hottest period was Aug 2006 to Jul 2007. Both of these 12 month periods were hotter than 1998. And these records were set during a deep solar minimum. Imagine what is yet in store!

 

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