Climate Change and Life on Earth

LIVING IN A WARMER WORLD

When we consider how global warming is impacting and will continue to impact on life on Earth, our fantasies inevitably focus on what life will be like for us and our children as individual members of a species living on Earth. The UN Intergovernmental Panel of Climate Change (IPCC) predicts that average temperatures on this planet will increase by up to about 6ºC by 2100 if no action is taken to reduce the emission of greenhouse gases from human activities. Media reports of this prediction are often accompanied by  images of people in swimsuits enjoying the sun, thus (intentionally or unintentionally) giving their viewers and readers cause to associate climate change with something we might actually enjoy. What person living through winters at temperate latitudes hasn’t at some point secretly wished for a little more of the “global warming” people are making such a fuss about?

On the face of it, a 6°C temperature increase does not sound like much. Many of us experience much greater temperature changes between summer and winter. What is really important in the IPCC prediction, however, is the seven-letter word hiding in the middle of the sentence: “average”. Imagine you have your feet in the oven and your head in the freezer. You might have a comfortable average temperature but you would most certainly find the extreme temperatures you experience at either end of your body uncomfortable. Global warming affects the Earth in much the same way. Relatively small (i.e. smaller than average) temperature increases are expected near the equator, while much greater temperature increases are predicted for polar regions. Temperature increases, no matter where they occur, influence life on Earth.

Already, the polar bear is being used as a poignant symbol of how global warming is threatening Arctic organisms. At the other end of the planet, scientists believe that the dramatic decline in numbers of Emperor Penguins seen in recent decades is related to climate change, and that continued temperature increases in this region could lead to the extinction of the species.

That polar bears and Emperor Penguins (and many other organisms) are endangered by climate change is not, in the first instance, a direct consequence of the warmer temperatures they are experiencing. All plants and animals have evolved to be able to survive under a range of temperatures and we know that both of these animals can survive hot summer days in city zoos, so it is not the temperature increase itself that is endangering polar bears and penguins. Rather, the threat arises from the changes in their habitats that are caused by the temperature increases. These habitat changes result in the disruption of ecosystems and, ultimately, of food webs. Thus, through its influence on their environment, global warming is very simply causing polar bears and penguins to starve.

The availability of food for both of these animals is dependent on the extent of the areas that are covered by ice. Polar bears hunt marine mammals from sea ice. Longer ice-free periods are occurring as a consequence of increased temperatures in the Arctic, and this is reducing the polar bears’ access to their food. Similarly, in the Antarctic, Emperor Penguins are dependent on small ocean animals (primarily krill) that feed on the algae that grow under the ice. With less ice, there is less substrate for the algae to grow on, less food for the krill and less food available to the penguins. Krill are considered to be a “keystone species” in the Antarctic ecosystem because so many other organisms (from penguins to baleen whales) are dependent on them for their food and, therefore, their survival. Thus, the reduction in ice cover – a direct consequence of global warming – has follow-on effects for the entire ecosystem and threatens a number of species found there.

The loss of ice caused by global warming, then, has profound effects on life cycles and ecosystem structure in polar regions. Similar examples (not with ice, of course, but where changes in habitat caused by climate change are disrupting or are predicted to disrupt ecosystems and threaten species) can be identified over the entire planet, both in the oceans and on land. For most organisms living on land, the most striking of these habitat changes relate to water. Here, the result of climate change is exposure to either much more or much less water than has been the case until now.

SEA-LEVEL RISE

We know that changes in climate are accompanied by changes in sea level. This happens for two reasons. First, warm water expands and the oceans simply fill more space in a warmer climate than a cold one. Second, and ultimately more important, is the fact that global temperature dictates how much of the water on the planet is in the form of ice. When glaciers on land melt, the waters they release make their way to the ocean and contribute to sea-level rise. Over geological time-scales of hundreds of millions of years, we know that the sea level on the planet has varied by hundreds of metres. Sea-level rise in response to the increasing global temperatures over the past several decades has already been recorded. The question now is how much sea-level rise we can expect in the coming years in response to continued warming.

In its 2007 report, the IPCC predicted a maximum global sea-level rise of 59 cm by 2100. However (and this point is seldom reported in the press), it also noted that there were some relevant factors that it did not take into account when it made this estimate because it simply did not have

the data. Data (especially concerning ice melting, which is happening faster than the IPCC predicted) is accumulating rapidly and most experts on ice and sea-level rise now believe that the UN estimate from 2007 is probably an underestimate of what we can expect to see in this century. There is still considerable discussion about how much the sea level will actually rise but suggestions of a metre or more are becoming more and more common in the scientific literature; and, of course, it is important to remember that the sea level will not stop rising in 2100. There is a considerable time-lag in the response of sea level to warming. Thus, even if we were to stop all human influence on the climate today, the sea level would continue to rise for several decades into the future. In other words, the decisions the present generation make about whether and how to combat man-made climate change have profound implications for future generations of our own and other species living on Earth.

INCREASED STORM ACTIVITY

Clearly, higher sea levels have serious implications for low-lying land regions. 45% of the human population and 75% of our megacities (more than 10 million inhabitants) are found in coastal regions and already there have been examples of small island communities in the Pacific and Indian oceans that have been forced to relocate to higher ground. However, it is not only the magnitude of sea-level rise itself that threatens our own (and other) species in these coastal habitats.

Storm surges cause major destruction of coastal habitat. One factor in determining the height of a storm surge is, of course, sea level. Another is the magnitude of the storm driving the surge. Hurricanes and typhoons (tropical cyclones) get their energy from the heat stored in the surface ocean. In fact, these storms only form over ocean waters where the surface temperature is at least 26.5°C. A warmer ocean thus increases the potential for these storms to develop and become more intense, compared to a colder ocean. Indeed, it has been demonstrated that there has been an increase in the intensity of hurricanes and typhoons in recent decades, corresponding to the period in which ocean temperature has been increasing. A longer time series is needed to argue unequivocally that global warming is the direct cause of this observed increase. However, the understanding of the relationship between ocean temperature and storm development makes the prediction of increased storm frequency and intensity in response to climate change robust and, as was shown so tragically when Hurricane Katrina hit New Orleans in 2005, the combination of storm surge and low-lying cities can be a fatal cocktail.

TOO LITTLE WATER

One of the most important factors influencing the distribution of plant life and structuring ecosystems on land is access to water. On a warmer planet, there is more water in the atmosphere. However, rainfall distributions also change dramatically. In general, we can say that the regions of the planet that are already “wet” (receiving much rainfall) will, as a consequence of the changing climate, get wetter; and those that are currently “dry” will get drier. The increase in drought in dry areas, especially, will have major implications for many species – including our own – in terms of their ability to find food. Access to food is already a problem in central Africa and climate change will only make this situation worse. The problem is not limited to Africa, however. In many regions around the world – in both developed and developing nations – water tables are falling and farmers are changing to crops that demand less water than those they have traditionally grown or dropping farming altogether.

Of course, as a region becomes drier, not only does it become more difficult for many plants to grow but the region also becomes more susceptible to fires. International attention has been directed towards the extensive and devastating wild fires that have occurred in recent years in Australia and western North America. Fires are natural phenomena and many factors influence their development. Therefore, a longer time series is needed before it can be established whether there is a causal link between the apparent increase in wild fires and climate change. Again, however, the prediction that fire will be a greater threat in some regions as a consequence of climate change is robust in that the risk of fire is increased as a region becomes drier.

CLIMATE CHANGE IS MORE THAN TEMPERATURE CHANGE

Almost everyone recognises that the increase in CO2 in the atmosphere caused by human activities influences climate by increasing the greenhouse forcing of the atmosphere. Few recognise, however, that this same increase in atmospheric CO2 is directly endangering many species in the ocean because it is making the surface waters of the ocean more acidic. The oceans cover 71% of the Earth’s surface and are in direct contact with the atmosphere. As the amount of CO2 increases in the atmosphere, more of it dissolves in the surface waters of the ocean. When CO2 dissolves in water, carbonic acid is formed and the acidity of the solution increases (this is why carbonated beverages are damaging to tooth enamel). Many organisms living in the ocean make calcium carbonate (chalk) and use it for shells or the structures they live in. Calcium carbonate dissolves under acidic conditions and it is predicted that even the comparatively small changes in ocean acidity that have occurred and are predicted to continue as atmospheric CO2 increases will have dramatic effects on a number of marine organisms.

One group causing considerable concern in this respect are the corals. Already, this group is under heavy pressure from the changing climate. It appears that many corals cannot tolerate warmer temperatures and coral “bleaching”, where corals lose their colour, often occurs following an unusually warm period. In addition, the structure of the type of calcium carbonate (aragonite) that corals form is particularly susceptible to increases in ocean acidity. Climate change is generally recognised as the greatest long-term threat to the Great Barrier Reef in Australia and some scientists have predicted that, if CO2 emissions continue unchecked, then well before the end of this century there will be no remaining regions in the world’s oceans that can support the healthy growth of corals.

LIFE ON EARTH AND THE CLIMATE SYSTEM

This chapter started by considering how climate change may affect life on Earth. It is only fitting to close by considering how the changes occurring in life on Earth may, in turn, influence and lead to changes in the climate. Our society is slowly accepting the fact that our species is changing the magnitude of the greenhouse forcing of the atmosphere by increasing the atmospheric concentrations of greenhouse gases. It should not come as a surprise, then, that other organisms in the Earth system also influence climate.

Through photosynthesis, plants remove CO2 from the atmosphere. We recognise some regions, such as the Amazonian rainforest, as being particularly important in terms of removing CO2 from the atmosphere and incorporating it into biomass. Scientists predict, however, that the combined effects of changing temperature and changing precipitation patterns resulting from man-made climate change will greatly reduce the extent of this rainforest area and, thus, reduce the capacity for CO2 uptake here.

Similarly, we know that, until now, the ocean has taken up between a third and half of the “extra” CO2 that we have emitted into the atmosphere. Much of this uptake is, again, due to photosynthesis. Some of the tiny plants, after having taken up this CO2, sink to the very deepest parts of the ocean, thus eliminating the possibility for the incorporated CO2 to return to the atmosphere. In a warmer ocean, the biological processes removing CO2 from the atmosphere will be diminished, both because there will be less plant activity (owing to a reduced mixing of nutrients from bottom waters to the surface in a more temperature stratified ocean) and because increased bacterial activity in the warmer waters will break down the sinking plant material before it reaches the deep ocean.

We see, then, that the influence of climate change on life on Earth will have feedback effects that influence the climate itself, and potentially exacerbate global warming. Because we are animals living in the atmosphere, we have a tendency to equate climate with air temperature and consider climate change as being a simple matter of changes in air temperature. As demonstrated here, however, nothing could be further from the truth. The air temperatures we experience are simply one component of the Earth’s intricate climate system, where not only the atmosphere, oceans and land but also every living organism plays a part. Predicting how climate change will influence the future of our species on Earth requires an understanding and appreciation of this marvellous system with its myriad of interactions.