In the news this week...

PAPER COMPARING THE MEDIEVAL WARM PERIOD, LITTLE ICE AGE AND THE 20TH CENTURY WARMING RATES

Using the climate system model 'FGOAL', TianJun et al., (2011) compared the differences between the rates of global warming between the Medieval Warm Period (MWP), Little Ice Age (LIA) and twentieth century. Multiple proxies were used in the model obtained from tree rings, ice cores, pollen and other records from thirty areas around the world, resulting in forcing data on volcanic events, solar insolation, nirous oxide, carbon dioxide and methane (Figure 1). Simulating changes on a global scale, the model indicated a warming during the MWP in all regions except the North Pacific. The model also demonstrated that warming was not uniform, with rates greater in the Northern hemisphere, than in the Southern hemisphere, and greater warming at higher latitudes. However, the overall warming during the MWP was not as strong as observations during the 20th century (Figure 2). During the LIA overall a global cold anomaly was observed in surface temperature, although mean surface temperature remained around +0.5° C in North-east America (although this is comparitively lower than during the MWP and 20th century anomaly).

Figure 1: Time series of forcing data used in the FGOAL model (Source Tian et al., 2011: 3084)



Figure 2: Annual mean surface air temperature anomalies for top to bottom (Medieval Warm Period, Little Ice Age and the 20th century).

Observations from the model support the concept of accelerated warming during the 20th century due to increased inputs of greenhouse gases into the atmosphere. This is evident in Figure 1, where there are significant increases in carbon dioxide, methane and nitrous oxide concentrations in the atmosphere from the Industrial Revolution in the mid-18th century, and is claimed by some (Crutzen, 2002) as the start of the 'anthropocene'.  


PALEOCLIMATIC RECORD SUGGESTS TIBETAN PLATEAU TEMPERATURES WILL DECREASE UNTIL 2068

A paper released by Lui et al., (2011) using palaeorecords from tree rings in the central-eastern Tibetan Plateau suggests that temperatures in the region will decrease until AD 2068 and will then begin to increase. Based on a 2485 year record, the tree ring record observes changes in temperature coincising with the MWP, LIA and 20th century warming, and cycles in temperature related to sunspot activity. Six cold and six warm events (half a standard deviation,±0.4°C, from the mean) were observed during the 2485-year period. Several significant cycles at 1324 year, 800 year, 190 year and 110 year were noted in the temperature series, and the cold events observed in the time-series coincided with periods of sunspot minimum. From the reconstructed temperature record, Lui et al., (2011) projects a decrease in temperature until 2068 reflecting as the earth enters a sunspot minimumn, followed by an increase in temperatures to 2088 (Figure 1).

However Lui et al.,'s (2011) study does not consider other forcings that influence global temperatures. Studies in previous posts have suggested that anthropogenic emissions of GHG's have accounted for a greater amount of the variability in global temperatures during the 20th century relative to insolation and volcanic activity. Therefore further research using multiple proxies covering a larger area is required to validate these findings. 

Figure 1: Reconstructed temperature record from a tree ring proxy in the central-eastern Tibetan Plateau over the last 2485years, and future changes projected due to sunspot variability.

Reference:

Liu, Y., Q. Cai, H. Song, Z. An and H. W. Linderholm (2011) 'Amplitudes, rates, periodicities and causes of temperature variation in the past 2485 years and future trends over the central-eastern Tibetan Plateau', Chinese Science Bulletin, 56: 2986-2994.

TianShun, Z., B. Lou, W. Man, L. Zhang and J. Zhang (2011) ' A comparison of the Medival Warm Period, Little Ice Age and 20th century warming simulated by the FGOALs climate systems model', China Science Bulletin, 56: 3028-3041.