Climate change

July 27, 2010

Climate change basics I – observations, causes and consequences

Filed under: Climate Change, Emissions Reduction, Global Warming — Barry Brook @ 5:27 pm


What is climate change? Observations, causes and consequences

Earth’s climate has always been dynamic and changeable. In the distant past there have been bouts of intense volcanic activity, periods when vast deserts spanned much of the globe, warm epochs when forests covered Antarctica, and glacial ages when much of Europe and North America were entombed under miles of ice. When large climatic changes occurred rapidly, a mass extinction of species was the result. Life later recovered, but this process inevitably took millions of years.

Just one species – humans – are now the agent of global change. As we develop our modern economies and settlements at a frantic rate, we have caused deforestation and fragmentation of natural habitats, over-hunting of wild species we use for food, chemical pollution of waterways and massive draw-downs of rivers, lakes and groundwater. These patently unsustainable human impacts are operating worldwide, are accelerating, and clearly constitute an environmental crisis. Yet the threat now posed by human-caused global warming is so severe that it may soon outpace all others.

Recent global warming is caused principally by the release of long-buried fossil carbon, by burning oil, natural gas and coal. Since the furnaces of the industrial revolution were first ignited in the late 18th century, we have dumped more than a trillion tonnes of carbon dioxide (CO2) into the atmosphere, as well as other heat-trapping greenhouse gases such as methane, nitrous oxide and ozone-destroying chlorofluorocarbons. The airborne concentration of CO2 is now 38 per cent higher than at any time over the past million years (and perhaps much longer – information beyond this time is too sketchy to be sure). Average global temperature has risen about 0.8°C in the last two centuries, with almost two-thirds of that warming having occurred in just the last 50 years. [play with some plots, here]

Complex computer simulation models of the atmosphere have been developed and refined for over 40 years. They are now sufficiently advanced that they can reproduce most of the major features of climate change observed over the last 150 years. Under a business-as-usual scenario, which assume a continued reliance on fossil fuels as our primary energy source, these models predict 1.8°C to 6.4°C of further global warming during the 21st century. There is also a real danger that we have reached or will soon reach tipping points that will cascade uncontrollably and take the future out of our hands. But much of the uncertainty represented in this wide range of possibilities relates to our inability to forecast the probable economic and technological development pathway global societies will take over the next few decades.

Year by year, our scientific understanding of climate science and responses of the Earth system continues to grow and mature.

In short, it remains within our power to anticipate many of the impacts of future global warming, and to 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 effort to avoid dangerous climate change?

Should we choose to take no effective action, we can expect increasingly severe consequences. For instance, beyond about 2°C of further warming, the Great Barrier Reef will be devastated. Extreme events will become much more frequent, such as storm surges adding to rising sea levels of many metres, threatening coastal cities. There is the possibility that a semi-persistent or more intense El Niño will set in, leading to frequent failures of tropical monsoonal rains which provides the water required to feed billions of people. Above 3°C, up to half of all species may be consigned to extinction because of their inability to cope with such rapid and extreme changes.

Worryingly, even if we can manage to stabilise CO2 concentrations at 450 parts per million (it is currently 387, and rising at 3 parts per million per year), we would still only have a roughly 50:50 chance of averting dangerous climate change. This will require a global cut in emissions of 50–85% by 2050 and certainly more than 90% for developed nations like Australia. Peak oil, global warming and long-term sustainability all require that we move rapidly to adopt sustainable, non-carbon energy sources, such as nuclear power and renewables (with the choice dictated largely by economic viability). Many credible studies show we can cost-effectively reduce greenhouse gas emissions IF the right policies are in place. For details of the principal mitigation and adaptation options that are available, see the 2007 IPCC Fourth Assessment Report and the Millennium Ecosystem Assessment.

Unfortunately, there is little evidence so far that we, as individual countries or as a global collective of humanity, are taking meaningful action. Indeed, ‘carbon intensity’ (expressed as gross domestic product per tonne of carbon emitted) in developed nations such as the United States and Australia has actually increased over the last decade, the global rate of emissions growth risen from 1% to 3% per year, and total carbon emissions from all sources now exceed 9 billion tonnes a year. China overtook the United States in 2006 as the single biggest greenhouse polluter and will be producing twice as much CO2 within little more than another decade at its present rate of economic activity.

This exponential growth in carbon-based energy, if sustained, will mean that over just the next 25 years, humans will emit into the atmosphere a volume of carbon that exceeds the total amount emitted during the 250-year industrial period of 1750 to 2000. Of particular concern is that long-lived greenhouse gases, such as CO2, will continue to amplify global warming for centuries to come. For every five tonnes added during a year in which we dither about reducing emissions, one tonne will still be trapping heat in 1,000 years.

It is a bleak endowment to future generations.

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