This is part II, on impacts of climate change. Be sure to read climate change basics I – observations, causes and consequences, and for more on pragmatic energy solutions, see here.
Climate change impacts on ice, rain and sea level
The term “global warming” says it all – a heating of the atmosphere right across the world. But that does not mean that the warming, or its impacts, will be the same everywhere. Regional and local differences can cause things to be worse, or better, depending on where you are.
One example of this unevenness is in the Arctic. Snow and ice melt over progressively larger areas and for longer periods as the temperature rises, causing the Earth’s surface to be duller. Bare rock, soil, vegetation and the open ocean are all much darker than bright ice, and so, just like the dark panels on solar hot water systems, absorb substantially more sunlight. This leads to greater heating, more melting, and so on – just one example of an amplifying feedback that can make global warming worse that it would otherwise be. There are many other such feedbacks, some of which remain poorly understood and could lead to more severe and more rapid warming than expected.
Perhaps the biggest regional impact of climate change faced by mid-latitude temperate regions (where most of the ‘developed nations’ are located), is, ironically, shifts in tropical-equatorial weather systems. Global warming causes the overturning tropical air masses that circulate in giant loops (called Hadley Cells and the Walker Circulation) to expand north and south. This has been recently shown to have happened already – up to 2° of latitudinal expansion over the last 30 years. Atmospheric heating also causes polar winds to whip around the Southern Ocean more rapidly. Together, these effects of global warming act to push rain-bearing mid-westerly weather systems further north and south. So instead of places like southern Australia being doused in rainfall brought in from the Indian and Southern Ocean, progressively more of this rain will be dumped uselessly over the sea, below the continental margin. This means less rainfall for Australia’s agricultural areas, as well other mid-latitude regions such as South Africa, the Mediterranean, Mexico and the western United States.
With less rain in these areas, the vegetation and soils will dry. In combination higher temperatures, the risk of bushfires intensifies. Heatwaves are the most dangerous culprits in this relationship. The 15-day March 2008 heatwave in Adelaide was, on the basis of the 20th century temperature record, a staggering 1 in 3000 year event. Yet under a mid-range projection of global warming (should no action be taken to quickly curtail carbon emissions), such an event would be an expected part of an average summer.
Such heatwaves and regular intensive fires also causes great stress to most species, leading to higher mortality, failed reproduction, and reduced body condition. These synergies, between water availability, hotter temperatures and changed fire regimes, are some of the primary reasons why unrestrained climate change is anticipated to lead to the extinction of an appallingly large fraction of our biodiversity. [climate change basics III will explain more about this]
Sea level rise is a more universal threat. When salty seawater mixes with fresh groundwater, the result is diluted seawater. A once usable water resource becomes worthless, with obvious impacts on coastal drinking and irrigation water supplies, as well as ecosystems which tap into aquifers. Severe storm surge events occasionally result in this exchange, but if these events are rare and do not encroach too far up the shoreline, the impacts are generally minor and localised. But what if the frequency of flooding events from the sea were to increase dramatically, and do so across the entire coastlines of heavily populated nations with expensive waterside properties or valuable low-lying agricultural land? That ominous threat is just what is anticipated due to climate change, and should therefore be a major concern to coastal planners and beachside residents alike.
There is clear evidence that sea levels have risen over the past century. Long-term records from a globally distributed network of reference tidal gauges show that sea levels rose about 20 cm from 1870 to 2004, correlating with a globally averaged rise in temperature of about 0.8°C. Since 1992, a satellite monitoring system has made regular and precise measurements of sea level, which show an accelerating rise over the last decade. If the Greenland and West Antarctic ice sheets hold together, the most recent estimates suggest another 50 to 140 cm of sea level rise this century. A worst-case scenario, now being predicted by some eminent scientists, is 3 metres by 2100 should the polar melt continue to accelerate. Yet, even 50 cm would be enough to make a 1 in 100 year storm surge event a yearly occurrence.
The need for action is urgent and our window of opportunity for avoiding severe impacts is rapidly closing. Yet the obstacles to change are not technical or economic, they are political and social.