Nature and Biodiversity

What does history tell us about climate change?


History of climate change

David F. Hendry
Director, Program in Economic Modeling, Institute for New Economic Thinking at the Oxford Martin School
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Future of the Environment

Life on Earth has survived great changes – there are many species alive today, some of which thrived in global temperatures much higher than now, so many stable levels of climate can support abundant life. But huge numbers of species went extinct from climate change. The Earth today is about 4–5°C warmer than in the last ice age, when Manhattan was under a mile of ice, so ‘small’ changes can matter greatly. Changes in climate, rather than actual levels within bounds, are what cause problems for life – adaptation is not instantaneous, as the ‘great extinctions’ emphasize.

Information from many sciences bears on the causes and consequences of both extinctions – as seen in the fossil record – and of climate change. Evidence from the composition of air trapped at the formation of rocks, the creation of coal and oil deposits from ancient tropical forests, long-run data in ice cores, and repeated glaciations revealed by movements of boulders from their origin, together reveal a vast range of past climates – from very cold to very hot (see, e.g., Hoffman and Schrag 2000), and from very dry to very wet.

Climate science

Many of the key ingredients of the climate picture are well understood, even though the Sun, Earth’s orbit and tilt, its atmosphere, land and worldwide ocean ‘conveyor belt’ system are complicated interacting processes. Energy balance models describe the incoming, outgoing and transfer of heat between the Sun and various levels of the atmosphere, land, and ocean, based on known physical (conservation) laws.

The four most important greenhouse gases are water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4), of which the first two are fundamental to life. The science of the effects of greenhouse gases in the atmosphere was established well before climate change became an issue. The National Academy of Sciences (2008) provides a clear explanation of why the Earth would be a frozen ball without a dense atmosphere, and also provides an excellent picture of how greenhouse gases receive and radiate at different wavelengths between ultraviolet and infrared, as well as their recent relative importance.

All four greenhouse gases are increasing at different rates from human activity, and have been since the origins of agriculture around 10,000–12,000 years ago, slowly at first and now much more rapidly. Such increases matter as they change the climate.

Climate change and 500 million years of extinctions

The fossil record reveals a background rate of extinctions, with the ‘timeline’ erupting at major geological boundaries. Figure 1 shows marine extinctions, where Cm=Cambrian; O=Ordovician; S=Silurian; D=Devonian; C=Carboniferous; P=Permian; Tr=Triassic; J=Jurassic; and K=Cretaceous.

Figure 1. Marine animal genera extinctions (percentage), millions of years ago


Source: Wikipedia.

The first mass extinction after the Cambrian occurred roughly 440 million years ago, probably due to global cooling (all dates are rounded here for simplicity). The second at about 375 million years ago followed the rapid spread of plant life on land, which reduced atmospheric CO2 by photosynthesis, so again led to cooling. The third and worst, at the Permian–Triassic (P/Tr) boundary about 250 million years ago, eliminated around 80–90% of ocean dwellers, and 70% of plants, animals, and insects (see Erwin 1996, 2006). Temperatures ended about 6°C higher than today, possibly from methane released in relatively shallow seas by magma flows creating the Siberian Traps (see Heydari et al. 2008 and Ward 2006).

The fourth extinction about 200 million years ago opened the ecological niche for dinosaurs in the Jurassic, possibly by release of gas hydrates inducing global warming. The fifth, and best known, major extinction occurred at the Cretaceous–Tertiary (K/T) boundary, roughly 60 million years ago when most dinosaurs (other than avian descendants) went extinct. It is plausibly attributed to an extra-terrestrial impact inducing rapid climate changes, although the formation of the Deccan Traps in India may also have played a role, with temperatures about 4°C higher than today (see Prothero 2008).

Mayhew et al. (2009) show that global biodiversity for both terrestrial and marine environments was related to sea-surface temperature, with biodiversity being relatively low during warm periods (also see Clarke 1993). All five mass extinctions are associated with climate change. A possible mechanism for warming extinctions being drastic is that the chemocline between oxygenated water above and anoxic water below rises with temperature (see, e.g., Riccardi et al. 2007). When the chemocline reaches the surface, archaea and anaerobic bacteria, such as green sulfur bacteria, proliferate and generate hydrogen sulfide (H2S), with toxicity comparable to hydrogen cyanide.

There is carbon isotopic evidence for the chemocline’s upward excursion during the end-Permian extinction, with a large increase in phototrophic sulfur bacteria replacing algae and cyanobacteria, consistent with huge loss of ocean life. As with CO2, hydrogen sulfide is heavier than air, so has a tendency to accumulate on the surface. The recent behaviour of the Black Sea reveals how quickly the chemocline can rise and the anoxic layer reach the surface, although that was due to fertiliser run-off, not global warming per se, so was more easily corrected (see Mee 2006).

Present evidence: Greenhouse gas increases

What is the evidence for the accumulation of CO2 equivalents in the atmosphere? Records at Mauna Loa in Hawaii from 1958 on by Charles Keeling show an increasing trend in CO2, with large seasonal variations, as seen in Figure 2 (a) (in parts per million, ppm), where panel (b) records the upward trend in annual changes in CO2, (c) shows global mean surface temperature deviations, and (d) shows ocean heat content deviations for 0–700 meters.

Figure 2. (a) Mauna Loa CO2 data; (b) annual changes in CO2; (c) surface temperature; (d) ocean heat


Sources: CO2 – US Weather Bureau and National Oceanic and Atmospheric Administration; temperature – Hansen et al. (2010); ocean heat content – Levitus et al. (2009).

Atmospheric CO2 levels of 172–300 ppm found in deep ice cores for the past 800,000 years now exceed 400 ppm (see Scripps CO2 Program 2010). Atmospheric methane has also doubled over that time (see Loulergue et al. 2008), with large recent increases in nitrous oxide as noted above. The atmosphere is a crucial blanket for life, but it is a thin one, as Figure 3 highlights.

Figure 3. The rising Sun highlights Earth’s thin layer of atmosphere


Source: NASA/SPL photo from the International Space Station.

Its composition can be changed by any steady accretion of gases (see, e.g., National Academy of Sciences 2008).

As a consequence, worldwide temperatures are slowly rising on a high-variance stochastic trend, buffeted by many influences as Figure 2 (c) shows (see Met Office 2008 and Pretis and Allen 2013). The ocean heat content is also rising. Both trends are consistent with the increased melting of Arctic ice. While oceans absorb CO2 and can probably absorb more (as can forests), that has adverse effects for marine life (see Stone 2007) – acidification slows the growth of plankton and invertebrates, which are basic to the ocean food chain. Lower pH levels could prevent diatoms and coral reefs from forming their calcium carbonate shells.

Moreover, while oceans absorb CO2 initially, some leaks back into the atmosphere – think sparkling water left unsealed. Since human activity is clearly responsible for the additional emissions, it is responsible for the ensuing temperature changes, accompanied by an increased rate of extinction (see WWF 2014).

Possible roles for economic analysis

There is a vast literature of economic analyses relevant to many aspects of climate change (see Goulder and Pizer 2006 for an overview). A key aspect is that there are uncertainties about the long-run evolution of climate, its relation to great extinctions, and present evidence about how rapidly global temperature changes respond to greenhouse gases – there are many ‘unknown unknowns’ (see, e.g., Weitzman 2009 and Ceronsky et al. 2011). But if you are unsure whether a basket covered with a cloth really has a solid base, be concerned about putting all your eggs in it – there are potentially serious risks to many life forms on Earth, so the economics of climate-change abatement matters.

Sudden changes are not just a more rapid version of gradual change, consistent with evidence from past great extinctions; rather, different forms of analysis are needed, as discussed in Hendry and Mizon (2014). Much of the stock of atmospheric greenhouse gases will persist for millennia, so climate change is not an easily reversible process, nor are extinctions of species, which permanently change biodiversity. There are many potential ‘tipping points’ for rapid changes that would be tantamount to shifts of global temperature distributions (see Lenton et al. 2008 and Katz 2010). My analogy in Hendry (2011) is of a car accelerating down a long hill with no means of stopping, facing an unknown drop an unknown distance away. Do you:

(a) ‘discount the future’ to decide when to jump later;

(b) hope that a rescue will somehow appear;

(c) jump out now when you may survive at the present speed?

The drop is the potential for mass extinction if mitigation is too delayed. As the conventional mathematics of intertemporal evaluations fails when distributions shift, the choice of a ‘discount rate’ is much less relevant than either organising technological or social rescues, or starting mitigation now.

Prizes for tackling climate change

One way to stimulate the first is to create a worldwide fund for large prizes on achieving pre-defined goals – such as inexpensive carbon capture and sequestration, efficient non-carbon-producing energy systems, an organism that eats CO2 to produce a fuel, etc. Past examples of successful, and inexpensive, prizes solving major problems are: the longitude problem, which Harrison’s chronometer solved; the Rainhill trials for which Stephenson invented his Rocket; or more recently, the Darpa Grand Challenge prize for a robotic vehicle crossing the Mojave Desert in less than 10 hours, which took just two years for four to finish the 241km race in that time; and the Ansari X Prize of $10m for a spacecraft to make two sub-orbital human flights, which was achieved in under a decade.

Humanity is not short of ideas, some of which may prove to be breakthrough technologies. Although patents also provide incentives for technological innovation on climate change, the ‘double externality’ stressed by Hall and Helmers (2010) (both environmental and knowledge externalities) make them less attractive, and it may prove difficult to protect intellectual property for major advances that resolve key forces driving climate change.

Past environmental negotiations

Concerning the second, social rescue, there are many relevant past negotiations about emissions reductions, most of which raised strong objections at the time with false assertions of the ‘high costs of abatement’ – yet with no notable impact on any country’s GNP. ‘Clean Air Acts’ led to major reductions in air pollution, but were accompanied by loud protests about the high costs. The so-called ‘prisoner’s dilemma in international collective action’ only holds for small countries not harming their own climate, but for Clean Air, many large countries legislated independently. Similarly for acid rain reduction from restricting sulfur dioxide (SO2) emissions, although again separate country legislation faced serious opposition. Parry et al. (2014) highlight the potential domestic health and efficiency benefits of unilateral CO2 reductions.

When supposedly inert chlorofluorocarbons (CFCs) transpired to be destroying the ozone layer by the release of chlorine from radiation, their elimination was agreed rapidly in the 1987 Montreal Protocol, helped by production being limited to a few players, and with a small ‘Multilateral Fund’ of $160 million to assist developing countries. The ozone layer already appears to be recovering slowly, but the replacements for CFCs, hydrochlorofluorocarbons, and hydrofluorocarbons seem to be powerful greenhouse gases, and Molina et al. (2009) propose building on the Montreal Protocol to mitigate that growing problem. In each case, there were losers in old industries but beneficiaries in new, and negotiators committed to mitigating climate change should highlight the advantages for early entrants of stimulating new industries.

Tradable permits, taxes, and energy efficiency standards

The second and third, starting mitigation now, mainly differ in their speeds of implementation. The theory of auctions (witness its huge impact on the UK’s 3G auction – see Binmore and Klemperer 2002), mechanism design, and taxation are all well developed, and despite their reputation as ‘weapons of financial mass destruction’, there is a potential role for modern finance theory in designing options and permits.

McKibbin and Wilcoxen (2002), for example, propose a combination of tradable permits and taxes to reduce emissions. Careful analyses of any proposals are essential to ensure that incentives are correctly aligned and reasonably costed, do not induce ‘substitution’ to least regulated areas, and don’t undermine living standards of the poorest (see Klemperer 2009). These are demanding, but not infeasible, requirements. Another fast impact policy is by mandating ‘best-practice’ standards for energy efficiency in transport, buildings, and appliances, as well as in all modes of energy production and distribution, with required improvements over time (see, e.g., Harvey 2014). Nevertheless, cumulative emissions determine temperature, so policies that reduce emissions help, but are not a complete solution (see Allen et al. 2009).


Economic analysis offers many insights – externalities need to be either priced or regulated, and climate change is the largest ever worldwide externality. All approaches are affected by the possibility of abrupt changes and the resulting unknown uncertainty when distributions shift, making action more urgent to avoid possible future shifts. Adaptation is not meaningful if food, water and land resources become inadequate. Conversely, mitigation steps need not be costly, and could stimulate innovation. International negotiations are more likely to succeed if the largest players act first in their own counties or groups – also creating opportunities for their societies as new technologies develop.

Planet Earth will survive whatever humanity is doing – the crucial issue is the effect of climate change on its present inhabitants. It is a risky strategy to do nothing if there are potentially huge costs when the costs of initial actions are small. The obvious time to start is now, and the obvious actions are the many low-cost implementations that mitigate greenhouse gases (see Stern 2008 for a list) – just in case.

Published in collaboration with VoxEU

Author: David F. Hendry, Kt, is Director, Program in Economic Modeling, Institute for New Economic Thinking at the Oxford Martin School, and Professor of Economics and Fellow of Nuffield College, Oxford University.

Image: A woman holds her baby as they walk in front of a coal-fired power plant on the outskirts of Datong, Shanxi province, November 20, 2009. REUTERS/Jason Lee

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