Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 21

The impact of this mid-Holocene drying on water quality and aquatic
biodiversity in North Africa has been most strikingly exemplified by Kröpelin
et al .'s study of Lake Yoa in the Eastern Sahara (Kröpelin et al . 2008), one of the
very few sites in the region to retain standing water throughout the entire period.
Detailed analysis of a sediment core from Lake Yoa has shown that the lake was
sustained beyond the mid-Holocene desiccation by fossil groundwater recharge.
Between 4200 and 3900 BP, however, there was a rapid switch from freshwater
to saline conditions, with water conductivity rising to over 20,000 m S cm −1
leading to a decrease in lake productivity and the establishment of a salt lake
macroinvertebrate community dominated by the salt-tolerant hemipteran Anisops
and brine flies ( Ephydra ). Although it has not yet been established whether the
change in climate that caused these events was gradual or rapid (cf. Holmes
2008), the history of this site illustrates clearly how climate change can force
freshwater systems across critical ecological thresholds and cause comprehensive
changes in ecosystem regimes, with negative consequences for human society.
Centennial to millennial scale
Whilst changes in orbital forcing can explain most of the very long-term changes
in climate, palaeoenvironmental records indicate that climate during the Holocene
has also varied on shorter scales. Explaining natural variability on these is less
easy than for multi-millennial timescales as mechanisms are not so clearly
understood. Some of the variability can be due to random fluctuations in the
climate system, some to the internal behaviour of the coupled ocean-atmosphere
system and some to external influences, especially to variability in solar activity.
Solar activity varies on the very well-known 11-year (Schwabe) and 22-year
(Hale) cycles now accurately measured by satellite radiometry (Fröhlich & Lean
1998) to the 87-year (Gleissberg), 210-year (Suess) and longer 2200-year
(Hallstatt) cycles. Evidence for the longer cycles can be obtained from
measurements of 14 C through the Holocene as solar activity, documented since
1610 from telescopic observations, has been shown to match closely the 14 C
variability recorded by tree rings over the same time period (Stuiver & Braziunas
1993). Moreover, 10 Be recorded by ice cores (Beer et al . 1990) also varies with
solar activity, and these two isotopic measures can therefore provide proxies for
solar variability back through the whole Holocene period.
The problem for climate science is understanding how very small changes in
solar output (c. 0.1% over an 11-year Schwabe cycle) can cause significant
fluctuations in climate. However, the variability is strongly wavelength dependent
with values fluctuating by more than 100% in the UV part of the spectrum (Beer
& van Geel 2008). Moreover, model studies have shown that there are potential
amplifying processes in the atmosphere that enable shifts in spectral solar irradiance
of this magnitude to cause shifts in tropospheric circulation systems that could
cause a significant response by the climate system (Haigh & Blackburn 2006).
Analysis of lake sediment records often reveals evidence of cyclical changes
that occur with the same periodicity as solar cycles, but establishing definitive
relationships between the two is often undermined by the relative inaccuracy and