A Variable Sun

What could have caused the droughts?

We have so far seen some pretty compelling evidence of persistent droughts, but nothing on what could have caused it. Unlike the collapse itself, the timing of the dry intervals may actually be easier to explain...

Drawing on the findings from the Lake Chichancanab sediment core, Hodell et al. (2001) argued that the droughts were actually part of a cyclical pattern that occurred every 208 years on average. They linked this to subtle variations in solar activity that remarkably followed an almost identical 206-year cycle.  Changes in the number of sunspots and the sun’s brightness were documented in tree-ring records of cosmic-nuclide production, whereby fewer cosmogenic nuclides (including ¹⁴C and ¹⁰Be) are produced when solar activity is higher.

When compared to the δ¹⁸O records, these bi-centennial oscillations seemed to be in anti-phase for the past 2.6 millennia, i.e. implying that during times of greater solar activity, droughts were prevalent. This corresponded well with discontinuities in the Maya cultural evolution.

And this doesn’t seem to be a one-off; other δ¹⁸O records in sediment cores taken from the nearby Lake Punta Laguna show the same thing (Curtis et al., 1996). There are also other records from around the world too:

• Venezuela: Foraminifera sequences (another proxy for palaeoclimate) from the Cariaco Basin were affected by regional upwelling and intensity of trade winds, attributed to a 200-year cycle of solar forcing (Peterson et al., 1991)
• Sahel region: Evidence of drought (not surprising since the climates of the Yucatan and Sahel are linked by the seasonal migration of the Atlantic ITCZ)
• Equatorial East Africa: Similar relationship between lake sediment records and solar activity for the past 1,100 years (Stuiver and Braziunas, 1993)
• Oman: Similar oscillations in climate recorded in a stalagmite, around the same time (Kerr, 2001). 

But you may ask: how can such small variations in solar activity cause such large E/P shifts in the Yucatan? Although it is uncertain, it may be via an amplifying mechanism. Possibilities include:

• UV changes that can affect ozone production and the temperature structure in the stratosphere
• Cosmic ray variability that can affect cloud formation and therefore precipitation
• Changes in solar output that can affect global mean temperature, convection and intensity of the Hadley circulation in the troposphere, in turn affecting rainfall (actually implied by sensitivity experiments  conducted with global circulation models (GCMs) - Hodell et al. (2001)).

Other factors?

It is very much possible that solar activity could have triggered the droughts, but it may also have been a combination of a variable sun and ocean current changes. Additionally, a restricted northward movement of the ITCZ that may occur when the trade winds are displaced further south could have contributed to the droughts, since convergence and cloudiness would be reduced (Brenner et al., 2001). This may be a non-linear response to solar output changes or due to other mechanisms. The change in climate may also have been induced by shifts in the El Niño-Southern Oscillation (ENSO) tropical air mass (McAnany and Negron, 2010). Fact is, climate is inherently complex, and most of the time there is more than one cause of climate change due to the feedbacks. 


References:
McAnany, P.A. and T.G. Negron (2010) 'Bellicose rulers and climatological peril?' In: McAnany, P.A. and N. Yoffee (eds) Questioning Collapse, Cambridge: Cambridge University Press, 142-175