Solar forcing

Having correlated drought with Classic Mayan collapse and highlighted it as a potential cause, it is then important to understand what prompted the drought itself. This understanding is significant as it potentially allows prediction of such events in the future and mitigation of their impacts.

One theory surrounding the cause of Classic Mayan drought was presented by Hodell et al. (2001). By undertaking a time series analysis of oxygen isotopes and gypsum precipitation, taken from an earlier core in Punta Laguna (Curtis et al. 1996), they revealed a recurrent pattern of drought with a dominant periodicity of 208 years. This cycle was said to be similar to the documented 206-year period in records of cosmogenic nuclide production (carbon-14 and beryllium-10) that is thought to reflect variations in solar activity. They concluded that a significant component of century-scale variability in Mayan droughts can be explained by solar forcing.

According to Hodell et al. (2001), the 206-year cycle of cosmogenic nuclide production is believed to reflect solar variability or a combination of solar forcing and oceanic response. It is said that periods of higher solar activity correspond to times of lower cosmonuclide production. It was found that the Punta Laguna δ¹⁸O and the ¹⁴C production show a significant relationship for the past 2000 years (Figure 1). It is apparent that higher E/P (δ¹⁸O) coincides with lower ¹⁴C production, implying that drought occurred during times of increased solar activity.

Figure 1: (A) Comparison of δ ¹⁴C (red line) and δ¹⁸O record from Lake Punta Laguna (black line). M, S, and W denote the Maunder, Sporer, and Wolf sunspot minima, respectively. (B) Bandpass filter centred at 208-years of the δ ¹⁴C record (red line) and δ¹⁸O signal from Lake Punta Laguna (black line). The blue line refers to Gamma-ray attenuation bulk density signal from Lake Chichancanab (Source: Hodell et al. 2001)

Hodell et al. (2001) state that the processes by which changes in solar activity cause climate shifts in Mesoamerica is not certain. However, it is known that in order to obtain a significant climate response from rather small variations in solar output an amplifying mechanism is required. Hodell et al. (2001) mention some suggestions for such mechanisms. One of these includes changes in the ultraviolet part of the solar spectrum, which affects ozone production and stratospheric temperature structure, and the effect of cosmic ray intensity on cloud formation and precipitation. Another possible mechanism has been found from sensitivity experiments conducted with atmospheric general circulation models. These suggest that changes in solar output may affect global mean temperature, humidity, convection, and intensity of Hadley circulation in the tropics.

Furthermore, Hodell et al. (2001) assert that mean annual rainfall on the Yucatan Peninsula alone varies by a factor of 5 over a distance of only 500 km between the semiarid northwest coast (500 mm/year) and the southern lowlands of northern Guatemala and Belize (2500 mm/year). Consequently, it is said that any solar-forced change in the strength or position of Hadley circulation or tropical convective activity would be expected to affect rainfall in the region.

Though the exact processes that allow increased solar activity to result in drought conditions are not yet fully understood, Hodell et al. (2001) revealed a strong correlation between the two. The strength of this correlation allows the conclusion that, through whatever mechanism, solar activity does play an important role in determining climate in the Mayan region. Consequently, these multi-decadal to multi-century-scale oscillations may well have initiated the detrimental impact on Maya food production and culture.