Wednesday, 7 March 2012

Alex Stubbings: Earth's Changing Climate

Editor: It is a pleasure to introduce to our readers, our new GfGD Blog Climate Change Correspondent - Alex Stubbings. Alex will be blogging on all matters related to climate change, climate change adaptation, and the important role geoscience can play in this discipline. Alex has a BSc in Environmental Geology from the University of Birmingham and an MSc in Climate Change from the University of East Anglia, as well as experience working in Bangladesh. 

 Climate Change (1) - Earth’s Changing Climate

Open up any geological text book, academic or popular science and one thing is immediately obvious: the climate of planet Earth isn’t stationary. Over vast swathes of time Earth’s climate naturally fluctuates between Icehouse and Hothouse, examples of which include: Neoproterozoic glaciations – so called Snowball/ Slushball Earth (Icehouse) and the Carboniferous (Hothouse). These are two examples outside of the Quaternary glacial cycle. In fact variations in Earth’s climate, along with the Great Oxidation Event, are linked to the evolution of life (also see here).

Source: Richard P. Hoblitt (USGS)
When it comes to climatic change over the eons what’s responsible? Volcanoes play a central role, for example the Siberian and Deccan Traps (at the End-Permian and End-Cretaceous). The actions of these flood basalts, over time, contributed significant quantities of greenhouse gases to the atmosphere. More recent examples include: Toba, 75,000 kyr (resulting in the human bottle neck); Tambora, 1815 (the following year was known as the year without a summer); and Pinatubo, 1992. The oceans play a central role also. They act as a ‘sleeping giant’ due, namely, to their thermal inertia which operates over timescales of 1000s of years, and ultimately reorganises thermohaline circulation systems. On shorter time scales thermal expansion can inundate low lying continental shelves. And lastly, macro-geologic processes: plate tectonics and continental drift result in climate change that plays out over time scales of 250 million years. For instance: Gondwanaland, Rodinia, Laurasia, Pangea, and the creation of isthmuses like that at Panama.

So it comes as nothing new that Earth’s climate changes, waxing and waning, from one state to another. This is but part of several processes that act over scales incomprehensible to us, but preserved forever in the rock record.

Microfossils (Source: Psammophile)
We know about changes in Earth’s climate not only from the rock record but from other palaeo-proxies: Speleothems, Ice cores, Varves, palynology, charcoal (analysis), dendrochronology (tree rings), macro and micro fossils, there is a wide range of proxies available to the geoscientist. Of course there are huge differences in resolution, spatial scale, and, importantly, uncertainty. Of the proxies listed above the most useful to re-tell Earth’s climate over the eons are macro and micro fossils. Think here about the End-Permian mass extinction or the End-Cretaceous, and the Palaeocene-Eocene Thermal Maximum –that’s where macro and micro fossils are most useful. The other proxies listed above deteriorate over time scales of 1000s >500,000 years: even ice cores!

Zooming in on our present situation and the geology explaining why we’re in another glacial period is very sound; here's one of my favourite papers to do with it. However, the most notable period of glacial activity is that of the Pleistocene. Since this epoch begun in Late-Quaternary time both hemispheres have seen vast sheets of ice accumulate. In fact we’re still in an Ice Age as there’s still ice present on Earth’s surface, however we’re in an interstadial: a warm period within an Ice Age. Other interstadials that Holocene workers know of include the Medieval Warm Period, where the Norse had settlements in Greenland.

The most recent stage of Quaternary time, the Holocene, is marked with stark climatic reversals. Starting from the Younger Dryas, which marked the termination of the last glacial period at 11,500 kyr, Earth’s climate has experienced marked changes, returning us to for a time to incipient glacial conditions. For instance the 8.2 kyr event in the Northern hemisphere, evidenced by marine drilling expeditions in the North Atlantic and ecosystem reorganisation over Europe and the Middle East; the End African Humid Period –which saw the intertropical convergence zone move northwards, resulting in the desertification of the Sahara; the widespread 4.2 kyr event which resulted in numerous civilisations collapsing throughout the low middle latitudes around the world.

Hopefully, as you can see climate has changed on Earth since time-immemorial and over different spatial scales. Climate has probably fluctuated like that seen in the Holocene over other periods of time, however due to the resolution and quality of data available to us we’re not able to differentiate these episodes.