The climate on our planet, over its billions of years of existence, has varied a lot—from times when the entire planet may have been enveloped by snow and ice (‘snowball Earth’) to times when tropical animals inhabited the polar regions. Even over the hundred-thousand years or so of Homo sapiens' tenancy, ice ages have come and gone. The most recent 8000 years or so, since the beginnings of agriculture and the first cities, however, have been unusually steady. Over this time, ice-core records show clearly that levels of CO2 in the atmosphere have been around 280 ppm give or take 10 ppm. CO2 is, of course, the principal ‘greenhouse gas’ in the atmosphere and the density of this ‘blanket’ plays a crucial, if complex, role in determining Earth's climate. Some have indeed argued that the beginnings of agriculture, and the subsequent development of cities and civilizations, is not a coincidence, but is a consequence of this unusual steadiness over many millennia.
Be this as it may, things began to change with the advent of the Industrial Revolution, which may be said to have begun with James Watt's development of the steam engine around 1780. As industrialization began to drive up the burning of fossil fuels in the developed world, CO2 levels rose. At first the rise was slow. It took about a century and a half to reach 315 ppm, moving outside the multi-millennial envelope. Accelerating during the twentieth century, the levels reached 330 ppm by the mid-1970s, 360 ppm by the 1990s and 380 ppm today. This change of magnitude by 20 ppm over only a decade has not been seen since the most recent ice age ended, ushering in the Holocene epoch, around 10 000 years ago. And if current trends continue, by approximately 2050 the atmospheric CO2 levels will have reached more than 500 ppm, roughly double pre-industrial levels.
There are long time lags involved here, which are often not appreciated by those unfamiliar with physical systems. Once in the atmosphere, the characteristic ‘residence’ time of a CO2 molecule is a century. And the time taken for the oceans' expansion to come to equilibrium with a given level of greenhouse warming is several centuries. It is worth noting that the last time our planet settled to greenhouse gas levels as high as 500 ppm was some 20–40 Myr ago, when sea levels were around 300 ft higher than today. The Dutch Nobelist, Paul Crutzen, has suggested that we should recognize that we are now entering a new geological epoch, the Anthropocene, which began around 1780, when industrialization began to change the geochemical history of our planet.
Such increases in the concentrations of the greenhouse gases which blanket our planet will cause global warming, albeit with the time lags just noted. As explained in David King's opening paper, the Intergovernmental Panel on Climate Change estimates that this warming will be in the range of 1.4–5.8°C by 2100. This would be the warmest period on Earth for at least the last 100 000 years. Many people find it hard to grasp the significance of such a seemingly small change, given that temperatures can differ from one day to the next by 10°C. But there is a huge difference between daily fluctuations, and global averages sustained year on year; the difference in average global temperature between today and the depth of the last ice age is only around 5°C.
As King emphasizes, the impacts of global warming are many and serious: sea level rise as mentioned above (which comes both from warmer water expanding and also from ice melting at the poles); changes in availability of fresh water (in a world where human numbers already press hard on available supplies in many countries); and the increasing incidence of ‘extreme events’—floods, droughts and hurricanes—the severe consequences of which are rising to levels which invite comparison with ‘weapons of mass destruction’.
This is why, in the context of the G8 Summit in the summer of 2005, the Royal Society took the unprecedented step of producing a brief statement on the science of climate change, signed by the science academies of all the G8 countries, along with China, India and Brazil. Making it clear that climate change is real, caused by human activities and with serious consequences, this statement called on the G8 nations to ‘Identify cost-effective steps that can be taken now to contribute to substantial and long-term reduction in net global greenhouse gas emission [and to] recognize that delayed action will increase the risk of adverse environmental effects and will likely incur a greater cost’.
So what should we be doing? One thing is very clear. The magnitude of the problem we face is such that there is no single answer, no silver bullet, but rather a wide range of actions must be pursued. Broadly, I think these can be divided into four categories: adapting to change; reducing wasteful consumption; sequestering at source some of the CO2 emitted in burning fossil fuels; and moving towards renewable sources of energy which do not put greenhouse gases into the atmosphere.
The papers in this volume derive from the Royal Society Discussion Meeting which, under the heading Energy for the Future, are addressed mainly to the fourth of the above categories, renewable energy. But the collection goes wider, beginning with scene-setting overviews by David King and by Rob Socolow, and also including a discussion of CO2 capture and storage by Sam Holloway. I like to catalogue our present and potential future sources of renewable, non-fossil-fuel energy in terms of the underlying fundamental forces of physics—nuclear, electromagnetic, gravitational—from which they derive (fossil fuels, of course, derive their energy from old sunlight, trapped by ‘photon-eating’ plants, and buried in the Earth; we currently consume about a million years of such ‘fossil energy’ each year). Thus categorized, nuclear forces provide fission, fusion and geothermal energy (the latter ultimately deriving from radioactive decay in the Earth's interior, without which the planet would have frozen several billion years ago). Tidal and hydroelectric energy are clearly gravitationally derived, and I guess wave and water also ultimately belong in this category. Solar and biomass energy sources take their energy from photons produced by nuclear reactions in the Sun, but the processes whereby human devices (based on physical or biological components) or plants capture these photons and make their energy available are basically chemical/electromagnetic, as is ‘hydrogen energy’. This entire panoply of possibilities is surveyed in the issue.
It is sobering to realize that the sources for the world's primary energy generation today are 80% fossil fuel (oil, gas, coal), 10% biomass (much of it simple deforestation, not renewably cycling CO2), 7% nuclear fission and only 3% all other renewables. We desperately need to be doing better.
- © 2007 The Royal Society