|Past Carbon Isotopic Events and Future Ecologies|
|Palaeoanalogue Carbon Isotope Excursions and Current Biosphere and Ecological Change as Guides to the 22nd Century and Beyond|
International symposium of the Geological Society with support of the British Ecological Society
The Earth has seen a number of (geologically) abrupt warming events. The Earth is currently embarking on a period of human-induced, carbon greenhouse gas induced warming. This warming is affecting ecological and human systems. However such abrupt (in the geological sense) carbon greenhouse gas induced climate change events are not unique in Earth's history: there have been a number of others and these too have affected past ecosystems. The most recent previous major such event took place in the early Eocene 55 million years ago (MYA) and is known as the Initial Eocene Thermal Maximum (IETM) or Palaeocene-Eocene Thermal Maximum (PETM). (Both 'IETM' and 'PETM' are mentioned as electronic searches of the literature need to be advanced searches including 'climate change' and both the terms 'IETM' and/or PETM. Some papers on palaeoclimate change (past climate change) use one or other of these terms.)
These warming events are global. Wherever on the planet 55 MY sediments have been found, the ratio of the carbon-12 to carbon-13 isotopes in them have been higher than the 12C to 13C before or after these events. So we know that these events were global and that they involved a release of 12C into the atmosphere that presumably ended up as carbon dioxide (CO2) which was then absorbed by algae which then ultimately settled on the sea floor to form part of the sediment. Because 12C isotope is central to these episodes they are known as 'carbon isotope excursion events.
The nature of these climate change events. These episodes (hence including the IETM / PETM) also have a number of similar characteristics (and many of these elements seem to becoming manifest with current global warming). These characteristics include:-
What seems to be happening? The mechanism of photosynthesis prefers 12C over 13C in CO2. The biosphere has various pools (or reservoirs) of carbon and so it those pools whose carbon has been through the photosynthetic process that is involved. Photosynthetically-sourced carbon pools include: vegetation (both living and decaying); fossil fuels; and methane clathrates.
So, what are the implications of these past events to current and near-future warming?
Abstract book briefly summarised
Actual text here: Cowie. J., & Cohen, A., (2010) Past Carbon Isotopic Events and Future Ecologies. Abstract Book for the symposium 2nd – 3rd November. Geological Society, London.
CIE events have been known since around 1990. The past 20 years has seen us begin to understand some of the key aspects of these events. That the Earth is currently going through a human-induced period of warming was hypothesised by Svante Arrhenius in 1896, and a consensus of international scientific opinion supporting this idea was formalised with the first Intergovernmental Panel on Climate Change (IPCC) report in 1990.
It is therefore arguably time to seek answers to some key questions. These might include:-
The IETM / PETM was caused by the relatively rapid release of thousands of petagrams (thousands of gigatonnes) of carbon. Some geological records suggest that the event's (geologically) abrupt onset may be less than a thousand years, while others a more gradual onset. The disagreements are largely an artefact of the sedimentary process that either truncate or stretch records.
There have been a number of CIEs in the Earth's history. The major CIEs coincided with severe global environmental perturbations. Despite major differences between the underlying conditions upon which past CIEs occurred, many of the key features with which the events were associated were broadly similar and included: severe global warming; acidification and deoxygenation of the oceans; extinction of marine and terrestrial species, and sudden shifts in the Earth’s climate and its hydrological cycle.
The Toarcian CIE and Oceanic Anoxic Event (OAE) occurred c. 183 million years ago in the early Jurassic. The impact of the event has been documented in many marine and terrestrial records of Toarcian age from around the World. The Toarcian CIE has features that are notably similar to those of the Palaeocene-Eocene Thermal Maximum and other CIEs, and suggest that the Toarcian CIE was caused by the addition of thousands of gigatonnes of isotopically ‘light’ carbon (
In terms of the amount of carbon involved in the Toarcian warming, annual rates of carbon addition may have approached those occurring today as a result of human activity. In contrast to the rapidity of its onset, records of the Toarcian CIE and of other comparable events show that the recovery of the Earth system occurred over much longer intervals of at least a few hundred thousand years. Furthermore there have been a number of Cretaceous carbon-isotope excursions (CIEs) that similarly record the addition of isotopically light carbon into the ocean–atmosphere system. Many of these excursions are accompanied by falls in the oxygen-isotope ratio of carbonate minerals or increases in the value of the organic geochemical proxy, TEX86: both imply geologically abrupt (millennial scale or less) warming of the surface ocean by several degrees.
There are still significant uncertainties in how the Earth’s climate will evolve in the future, and these uncertainties become very pronounced as the climate is pushed beyond its current regime. The geological carbon isotope records contain much evidence of abrupt changes in climate in the past. Is it possible that the Earth’s climate could ' runaway' or flip to a new 'hothouse' stable state under scenarios of increased greenhouse gas emissions? The notion of such abrupt climate changes, thresholds or so-called 'tipping points', is now a hot topic in climate change research. One of the greatest concerns is that reserves of carbon, in the form of methane hydrates (clathrates) stored in ocean sediments and beneath permafrost, could be destabilized by global warming. An event of this type is considered to be the likely explanation for one of the most pronounced CIE seen in the Earth’s geological record, the IETM / PETM. It can be seen as both an analogue for 21st and 22nd century global warming, and also a warning of the possibility of very strong feedbacks that could significantly amplify human-induced global warming.
Using current climate models, even allowing for the different shapes of the continents in the early Eocene, do not give results of sufficient warming to account for the ecological change we see from the early Eocene fossil record. It is likely that current computer models are too insensitive to greenhouse gas change.
During the IETM-PETM global temperatures increased by somewhere around 5°C, and regionally temperatures were ~5-9 °C higher at mid- to high-latitudes than before or after this CIE. The warming of high-latitude land bridges allowed mammals to disperse among the northern continents resulting in a profound, and permanent reorganization of mammalian faunas. The earliest perissodactyls (odd-toed ungulates), artiodactyls (even-toed ungulates), and euprimates ('true' primates) first appeared in North America, Europe, and possibly Asia during the IETM / PETM. Transient 'dwarfing' occurred in several mammalian groups, whereby some species become markedly smaller.
In palaeo-Great Britain, the geological record from Cobham documents a major vegetation change. A pre-IETM / PETM low diversity, fire-prone, herbaceous fern and woody angiosperm community is replaced with the onset of the CIE. Ferns are lost, fires cease, wetland plants increase (including swamp conifers and water ferns like Azolla and Salvinia) and a wider variety of flowering plants, including palms.
With regards to the marine environment, CIE's are marked by deep-sea acidification, but evidence for surface ocean acidification (deformation of calcifying phytoplankton) is not universally accepted. Sedimentological, ichnofossil, microfossil and geochemical evidence indicates widespread low oxygen conditions during the IETM / PETM and possibly other CIEs in coastal regions and epicontinental basins (New Jersey, New Zealand, North Sea, parts of Tethys), but also at mid-depths in the Southeastern Atlantic, where the Oxygen Minimum Zone expanded. The geographic and bathyal extent of hypoxia is not well defined. Eutrophication may have increased in marginal and epicontinental basins, whereas oligotrophy increased in open ocean environments, possibly linked to increasing ocean stratification.
In the present day, the increase in atmospheric CO2 since 1800 has increased the surface ocean CO2 concentration. Surface ocean pH has already fallen by 0.1 units, with a further 0.4 pH unit decrease projected for 2100. The most obvious biological effect will be in organisms calcified with CaCO3, where calcification will become more costly in energetic terms if it can occur at all, and calcified structures outside cells will dissolve. However, experiments in which calcification of marine organisms has been measured under future CO2 conditions have yielded variable results.
There is an additional uncertainty limiting our ability to predict human behaviour. In recent years there has been an increasing recognition that the impacts of climate on marine ecosystems and their resources interact with human responses to influence the outcome. In other words, humans are not just recipients of the impacts but part of the process leading to change.
On land we can examine contemporary and historic data on native species in relation to climate and this provides evidence for the impacts of recent climate change. It allows cautious prediction of changes in the coming decades, and provides parameters for modelling future impacts. The bulk of biological changes we see are in the direction expected under a currently globally warming climate.
Complications do arise in predicting future changes. Responses to climate are unlikely to be linear. There may be thresholds or so-called 'tipping points', step changes or inertia (delayed) effects, that all need to be considered. Furthermore, species are not immune to changes happening to prey, predators and competitors, from land use change, from exploitation, and from environmental pollution. Climate change is already projected to have a major impact on species, with, for both climate change and land use (habitat loss) reasons on average 15% to 37% of species committed to extinction by 2050 and between 1% and 43% of endemic species by 2100. Significant changes are also projected for the end of the century for some ecosystems and the services they provide.
Palaeoscience shows us that the Earth system has undergone abrupt and non-linear transitions in the past, and gives us valuable clues as to where to look for future vulnerabilities. Analysis of past abrupt climate changes show that some did carry early warning signals in the form of ‘critical slowing down’, whereas others were purely ‘noise induced’ (i.e. triggered by large natural variability) with no early warning. Analysis of recent climate data suggests that a new climate state is already beginning to emerge.
Current UK climate policy proposals as such that the UK would have played its 'fair share' in the necessary reduction of its per capita carbon footprint, namely a reduction such that we have a 50% chance of holding average global temperature rise below 2°C* were all countries to participate.
Ecosystems provide both economic goods and services not generally accounted for by economists. Since the UN Millennium Ecosystem Assessment formalised the definition of 'ecosystems services' there has been increasing interest by policy makers to capture all ecosystem benefits from 'ecosystem services', as demonstrated last month in the UK. Models suggest that should the planet be subject to a (theoretical) 3°C or 4°C warming from present day temperatures we would see major biome change broadly comparable in degree of spatial dislocation to that of glacial-interglacial transitions. They also suggest critical change to those biomes in areas that currently provide us with traditional economic benefits, such as (importantly) food, as well as trap carbon. Consequently, both the direct (normally accounted), as well as the less-visible, economic costs of the climate-induced disruption to ecosystem services globally would be profound. This lends urgency into nailing the question as to whether or not current warming is likely to trigger an event analogous to carbon isotope excursion (CIE) events of the past? If there is a meaningful risk and hazard – and depending on the likely timescales involved – then societies, in addition to attempting to curb greenhouse emissions, may want to pay more attention to climate adaptation policies.
*Personal note: on the above summary of the abstract booklet. While the above is a summary of the booklet, on a personal basis for some years I have doubted that we can achieve the necessary emission reductions in line with current (November 2010) UK policy goals to keep us below 2°C warming above pre-industrial temperatures, which many consider the limit of ecologically 'safe' warming. And, in 2009, I posted my view that we will most likely exceed this limit. This lends greater urgency to the above conclusion that we need to spend more effort on adaptation to climate change (as so far little attention internationally has been spent on adaptation with nearly all the effort on emission reduction which has not been achieved).