Consequences ... what can we expect in a warming world?
The seas will rise
Some other consequences are examined on the next page
Here is a list of ten consequences that are fairly well researched
Nothing we say about the future is final - until it isn’t the future any more; but these broad projections are probably beyond doubt. The trouble is: most of us want a bit more detail than this - which for now we don’t have. Each one of these is still remediable if we decide to act in time.
1. Sea level will rise. Each time a major revision is made to these estimates, the rate and onset of rise is a bit more alarming. That is partly because the polar ice has been giving us some surprises, and partly because it is such bad news we prefer listening to the least dire versions until they are absolutely refuted. The main reason these predictions are uncertain is that we know too little about the behaviour of the great ice sheets under a large greenhouse forcing.
2. Weather patterns will change. Equatorial zones will become wetter, with more violent storms and floods; the sub-tropical zone will expand and become drier - bad news for Africa, Australia, the Mediterranean, USA, China, India & elsewhere. Large weather phenomena such as hurricanes will be more energetic because of warming seas. In high latitudes, there will be more precipitation as rain, and less snow - bad news for places that depend on summer melt for water supplies. No one yet knows exactly how the big irregular oscillations - El Nino, the Asian monsoon & others will behave.
3. The poles will heat much more than the rest of the world. Ice is being lost from the great ice sheets - Greenland,the Antarctic and the arctic sea-ice at rates that, not long ago would have seemed incredible. The loss of these great refrigerators of the world will accelerate the heating of the rest of the globe - for two reasons: first, they act like big mirrors, reflecting solar energy back to space; second, they generate cold air & water that cools the high latitudes.
4. Tropical forests will probably die, along with coral reefs. Most specialists believe the great Amazon basin forest will disappear because events like the 2005 drought and fires will return frequently. A similar fate will probably finish what is left of the south-east Asian forests. This is not inevitable. The fate of the forests will be a combination of poorly managed exploitation and ecosystem change.
5. Oceans will be acidified by dissolved CO2, making large areas lifeless. This geo-chemical effect is already underway, can be measured very precisely, and many of its consequences calculated exactly. Because of cascading effects on ocean food-chains, some specialists believe this is one of the most serious potential effects of the CO₂ surge.
6. Hundreds of millions of people will be short of fresh water. Even though rainfall will become heavier in some places, it is easy to identify heavily populated regions where it will decline, as has happened already to every one of the Australian capital cities except Darwin & Hobart, the smallest. This problem, manageable but expensive in Australia, will be disastrous in most of the world where no ready solution exists. Perhaps several hundred million people (both urban & rural) depend for water on rivers fed by seasonal glacial meltwater. For the time being, while glaciers retreat, summer flows are increasing, but in coming decades they will sharply decline.
7. The distribution of important human and animal diseases will change. This is happening already and will become a major problem as microbes and vectors move with their habitats. Possibly exotic or unknown pathogens will emerge to exploit new environments.
8. Food production will be severely disrupted by droughts and disappearance of irrigation water. Some of the most intensively cultivated land on Earth will suffer major productivity decline, and it is not the case that new land will be available to replace it. All our traditional crops are adapted to quite particular growing conditions. Most of them will not be as productive in a hotter world.
9. Societies are likely to respond to severe stresses in violent ways. We don’t need to be very perceptive to know that unprepared people have always resorted to coercion when their survival is threatened. It would be naive to suppose this will be any different. Vast numbers of people will be motivated to move, invade, commandeer resources, throw their weight around and abandon their liberty in favour of men of action.
10. The world’s biodiversity will be greatly reduced. This claim, so often heard it has lost much of its power to dismay, means many things. But the one thing that hardly needs interpreting is that our children and their children will live in a poor world, reduced in variety and possibility, a world full of the marks of destruction, rather than the rich one we knew.
Rising seas ...
The oceans will certainly rise. We know this because it is one of the changes that’s already underway ... but there’s a catch ...
This is exactly when we’d most like to know the details - and we don’t have them.
On the left is John Church’s composite study of twentieth century sea-level change. Even without a calculator, you can see there’s an upward trend in this wriggly line. The red bit is data provided by the TOPEX & Jason satellite-born altimeters - incredibly precise instruments that can measure the height of the sea surface to an error of a couple of millimetres. According to them, the current rate of rise is 3.4mm/yr. The rate in 1900 was 1.1mm/yr - so the rate has tripled. But what will it do next?
That depends on how the ice sheets of Greenland and West Antarctica respond to the coming warmth. We can get some idea about this by studying what they did at the end of the Eemian - the last warm interglacial, 125,000 years ago, when the world got maybe a degree warmer than now. But that was a natural warming, not driven by a strong greenhouse forcing, and much slower than now. The fact is, we have no idea whether these great ice bodies can disintegrate fast or not.
Consequently, estimates of sea-level rise for 2100 range from 0.5m to over 2 metres. Everything depends on how fast heat can be transferred to the ice. A marine ice sheet like the West Antarctic, can be heated from below by a warming Southern Ocean (something which seems to have happened in the past) and Greenland can apparently admit large amounts of surface meltwater to its interior via fissures that transfer meltwater all the way to glacial bedrock - something completely unexpected until it was observed a few years ago.
We know that big ice sheets can make prodigious quantities of meltwater fast. Look at the second chart on the left. It shows the rise of the ocean between the end of the last ice-age, 18,000 years ago and the start of Holocene warmth, 8,000 years ago. In those 10,000 years, the sea rose 120m - an average rate of 1.2cm/yr. But for the 400-500 year period labelled “meltwater pulse 1A”, it rose four times this fast - a total of 20m (40-50cm/yr). In four centuries, three times the volume of the Greenland ice sheet entered the ocean.
This water didn’t come from Greenland - but the record tells us that, under some circumstances, big ice sheets can melt fast. Stefan Rahmstorf, a leading expert on this, in a study in 2009, gave a range of 98-130cm for 2100 (see third chart on the left); but has said since that a rise closer to 2m is not out of the question.
However, it can be quite misleading to focus on sea level predictions for the end of the century - for if we don’t know where the tide will be then, we do know it’s not stopping. In fact all projections show the rate accelerating for several centuries to come. Look at the bottom chart - a detailed reconstruction of sea-level change during the Eemian, culminating in an excursion to 9m above present, for perhaps a couple of centuries. The maximum rate of rise recorded here was 2.5m/century (about 8 times the current rate) and this water DID come from Greenland & West Antarctica, under a weaker forcing than we have now.
In summary, inevitable sea-level rise is a colossal problem we haven’t really come to terms with: our information is still too incomplete, and we hardly appreciate yet how much damage it will do for hundreds of years to come. This is probably the single biggest threat to civilised life that we can foresee.
Ice & sea
There are two reasons why the sea will rise in future: warming the ocean makes it expand, and warming the poles melts the polar ice caps. There's about 33,000,000 km3 of ice on land at the poles. Melting it all would raise the sea about 75m.
One of the things we learned from studying the past is that the world has been in that state before. 35 million years ago, and for ages before that, there was no Antarctic ice sheet, so there is nothing new about all the world's water being in the global ocean.
The questions that keep specialists wondering are: whether it could happen again ... and how fast ... and what it would take ... and how we can find out.
Very clever measurements are beginning to reveal how the two vulnerable ice sheets (Greenland & West Antarctica) are responding to the 0.8℃ of warming so far. These graphs were made from ground-based calculations of snow-fall and melt rate; air-borne & satellite radar surveys; and the GRACE gravimetry satellites. Together, they show a downward trend for both ice sheets. The records are not quite long enough yet to give the trends a mathematical description ... that would allow better predictions. This should be possible within a decade.
How much ice is melting?
It's hard to get an idea of the enormous quantities involved. These structures are so huge and most of us have never seen them, and it's happening so far away.
A gigatonne of water is the same as a cubic kilometre, and it takes about 360 km3 to raise sea-level 1 mm.
In a major review of the state of knowledge published in 2012, the following summary estimates were given for the period 1992-2011:
* Net loss from the Greenland ice sheet 142 Gt/yr
* From the West Antarctic ice sheet 65 Gt/yr
* From the Antarctic Peninsula 20 Gt/yr
* The great East Antarctic ice sheet was reckoned to be in approximate mass balance.
* In total, the contribution of the ice sheets to global sea-level rise was about 0.6mm/yr
There are significant uncertainties ... the most precise satellite observations have been going less than a decade, and there have been things to work out about their interpretation. The crucial thing is, if the rate of mass loss turns out to be increasing exponentially (difference between the red, green or black lines in the charts above) it will accelerate rapidly over the next couple of decades, and conceivably trigger some geophysical instability in the ice body causing the kind of collapse that apparently took place 14,000 years ago during MWP 1A.
Managing the effects of continuously rising seas on coastlines is fiendishly difficult - which is why coastal management policy in most parts of the world is feeble and inconsistent with the known facts. Many administrators are acting as if they didn't know what to do.
There's a short video about this here
What will it be like in a warmer world?
We can say something about this because the warm future has already begun. There are three sources of knowledge about the future climate:
• observing current trends;
• searching the record of the past; and
• running climate simulations on computers.
Each method has its strengths & weaknessess. We don't know it all. But certain things are pretty clear.
1. The sea will rise. How far and how fast, we don't yet know.
2. Species will be lost. We are losing them now, but the future will be worse.
3. More heat in the air and sea means more enrgy in storms, and heavier rain.
4. Expansion of the dry sub-tropical belts means drought in those zones.
5. The food production system will have to adjust - exactly how, we don't know.
6. The ocean will be more acidic, with multiple effects on ocean life.
