The world’s climate is inherently dynamic and changeable. Past aeons have borne witness to a planet choked by intense volcanic activity, dried out in vast circumglobal deserts, heated to a point where polar oceans were as warm as subtropical seas, and frozen in successive ice ages that entombed northern Eurasia and America under miles of ice. These changes to the Earth’s environment imposed great stresses upon ecosystems and often led to mass extinctions of species. Life always went on, but the world was inevitably a very different place.
We, a single species, are now the agent of global change. We are undertaking an unplanned and unprecedented experiment in planetary engineering, which has the potential to unleash physical and biological transformations on a scale never before witnessed by civilization. Our actions are causing a massive loss and fragmentation of habitats (e.g., deforestation of the tropical rain forests), over-exploitation of species (e.g., collapse of major fisheries), and severe environmental degradation (e.g., pollution and excessive draw-down of rivers, lakes and groundwater). These patently unsustainable human impacts are operating worldwide, and accelerating. They foreshadow a grim future. And then, on top of all of this, there is the looming spectre of climate change.
When climate change is discussed in the modern context, it is usually with reference to global warming, caused by anthropogenic pollution from the burning of fossil fuels. Since the furnaces of the industrial revolution were first ignited a few centuries ago, we have treated the atmosphere as an open sewer, dumping into it more than a trillion tonnes of heat-trapping carbon dioxide (CO2), as well as methane, nitrous oxide and ozone-destroying CFCs. The atmospheric concentration of CO2 is now nearly 40% higher than at any time over the past million years (and perhaps 40 million years – our data predating the ice core record is too sketchy to draw strong conclusions). Average global temperature rose 0.74°C in the hundred years since 1906, with almost two thirds of that warming having occurred in just the last 50 years.
What of the future? There is no doubt that climate predictions carry a fair burden of scientific ambiguity, especially regarding feedbacks in climatic and biological systems. Yet what is not widely appreciated among non-scientists is that more than half of the uncertainty, captured in the scenarios of the Intergovernmental Panel on Climate Change, is actually related to our inability to forecast the probable economic and technological development pathway global societies will take during the twenty-first century. As a forward-thinking and risk averse species, it is certainly within our power to anticipate the manifold impacts of anthropogenic climate change, and so make the key economic and technological choices required to substantially mitigate our carbon emissions. But will we act in time, and will it be with sufficient gusto? And can nature adapt?
The choice of on-going deferment of action is potentially dire. If we do not commit to deep emission cuts (up to 80% by 2050 is required for developed nations), our descendents will likely suffer from a globally averaged temperature rise of 4–7°C by 2100, an eventual (and perhaps rapid) collapse of the Greenland and the West Antarctic ice sheets (with an attendant 12–14 metres of sea level rise), more frequent and severe droughts, more intense flooding, a major loss of biodiversity, and the possibility of a permanent El Niño. This includes frequent failures of the tropical monsoon, which provides the water required to feed the billions of people in Asia.
Indeed, the European Union has judged that a warming of just 2°C above pre-industrial levels constitutes ‘dangerous anthropogenic interference with the climate system’, as codified in the 1992 United Nations Framework Convention on Climate Change. Worryingly, even if we can manage to stabilise greenhouse gas concentrations at 450 parts per million (it is currently 383 ppm CO2, and rising at 3 parts per million per year), we would still only have a roughly 50:50 chance of averting dangerous climate change. Beyond about 2°C of warming, the likelihood of crossing irreversible physical, biological and, ultimately, economic thresholds (such as rapid sea level rise associated with the disintegration of the polar ice sheets, a shutdown of major heat-distributing oceanic currents, a mass extinctions of species, and a collapse of the natural hazards insurance industry), becomes unacceptably high.
Unfortunately, there is no evidence to date that we are taking meaningful action to decarbonise the global economy. In fact, it is just the reverse, with a recent work showing that the carbon intensity of energy generation in developed nations such as the US and Australia has actually increased over the last decade. Over the last decade, the world’s rate of emissions growth has tripled, and total CO2 emissions now exceed 30 billion tonnes a year. China overtook the US in 2006 as the single biggest greenhouse polluter, and within a decade, it will be producing twice as much CO2. This remarkable rate of growth, if sustained, will means that over just the next 25 years, humans will spew into the atmosphere an additional volume of CO2 – greater than the total amount emitted during the 150 year industrial period of 1750 to 2000! Of particular concern is that long-lived greenhouse gases, like CO2, will continue to amplify global warming for centuries to come. For every four tonnes added during a year in which we prevaricate about reducing emissions, one tonne will still be trapping heat in 500 years. It is a bleak endowment to future generations.
Nature’s response to twentieth-century warming has been surprisingly pronounced. For instance, ecologists have documented numerous instances of shifts in the timing of biological events, such as flowering, emergence of insects, and bird migration occurring progressively earlier in the season. Similarly, many species, including insects, frogs, birds and mammals, have shifted their geographical range towards colder realms – towards higher latitudes, upwards in elevation, or both. Careful investigations have also revealed some new evolutionary adaptations to cope with changed climatic conditions, such as desiccation-tolerant fruit flies, and butterflies with longer wings that allow them to more readily disperse to new suitable habitats. On the other hand, some sensitive species have already been eliminated by recent climate change. For instance, the harlequin frog and golden toad were once found in abundance in the montane cloud forests of Costa Rica. But in the 1980s they were completely wiped out by a fungal disease, which flourished as the moist forests began to dry out: a drying caused by a rising cloud layer that was directly linked to global warming.
These changes are just the beginning. Under the current business-as-usual scenario of carbon emissions, the planet is predicted to experience five to nine times the rate of twentieth-century warming over the next hundred years. An obvious question is, will natural systems be able to continue to keep pace? There are a number of reasons to suspect that the majority will not.
Past global climate change characteristically unfolded over many millennia, whereas current anthropogenic global warming is now occurring at a greatly accelerated rate. If emissions are not checked, a level of planetary heating comparable to the difference between the present day and the height of the last ice age, or between now and the age of the dinosaurs (when Antarctica was ice free), is expected to unfold over a period of less than a century! When such catastrophically rapid changes in climate did occur, very occasionally, in the deep past – associated, for instance, with a large asteroid strike from space – a mass extinction event inevitably ensued. Most life just could not cope, and it took millions of years after this shock for biodiversity to recover. It has been estimated that 20 to 60 per cent of species might become extinct in the next few centuries, if global warming of more than a few degrees occurs. Many thousands (perhaps millions) will be from tropical areas, about which we know very little. A clear lesson from the past is that the faster and more severe the rate of global change, the more devastating the biological consequences.
Compounding the issue of the rate of recent climate change, is that plant and animal species trying to move to keep pace with the warming must now contend with landscapes dominated by farms, roads, towns and cities. Species will gradually lose suitable living space, as rising temperatures force them to retreat away from the relative safety of existing reserves, national parks and remnant habitat, in search of suitable climatic conditions. The new conditions may also facilitate invasions by non-indigenous or alien species, who will act to further stress resident species, as novel competitors or predators. Naturally mobile species, such as flying insects, plants with wind-dispersed seeds, or wide-ranging birds, may be able to continue to adjust their geographical ranges, and so flee to distant refugia. Many others will not.
A substantial mitigation of carbon emissions is urgently needed, to stave off the worst of this environmental damage. But irrespective of what we do now, we are committed to some adaptation. If all pollution was shut off immediately, the planet would still warm by at least a further 0.7°C.
For natural resource management, some innovative thinking will be required, to build long-term resilience into ecosystems and so stem the tide of species extinctions. Large-scale afforestation of previously cleared landscapes will serve to provide corridors, re-connecting isolated habitat patches. Reserves will need to be extended towards cooler climatic zones by the acquisition of new land, and perhaps abandoned and sold off along their opposite margins. Our national parks may need to be substantially reconfigured. We must also not shirk from taking a direct and active role in manipulating species distributions. For instance, we will need to establish suitable mixes of plant species which cannot themselves disperse, and translocate a variety of animal species. It may be that the new ecological communities we construct will be unlike anything that currently exists.
Such are the ‘unnatural’ choices we are likely to be forced to make, to offset the unintended impacts of our atmospheric engineering. Active and adaptive management of the Earth’s biological and physical systems will be the mantra in this brave new world. Truly, the century of consequences.