What We Do and Don’t Know About Climate Change

As I said in the last post, the science behind climate change has been in the news recently, which brings the side benefit of having New Scientist publish a series of articles on climate change for our education. You may not be able to access the articles without registering with them (for free), but they’re very short in any case and I’ll summarize them here. Because they’re very short though, be warned that they’re probably slight-to-gross oversimplifications, so don’t take any of this as whole, perfect truths. 

This ended up being super duper long – feel free to just skim over the titles and read more only if you’re interested. I think the important thing to note is that we don’t know everything – and we likely never will. That applies to every field of science though, as the final article eloquently explains. Climate change science has been brutally politicized, but that shouldn’t distract people from the facts.

Climate known: Greenhouse gasses are warming the planet

From melting glaciers and earlier springs to advancing treelines and changing animal ranges, many lines of evidence back up what thermometers tell us – Earth is getting warmer. Over the 20th century, the average global temperature rose by 0.8 °C

Studies of Earth’s past climate tell us that whenever CO2 levels have risen, the planet has warmed. Since the beginning of the industrial age in the 19th century, CO2 levels in the atmosphere have increased from 280 parts per million to 380 ppm. Satellite measurements now show both that less infrared of the specific frequencies absorbed by CO2 and other greenhouses gases is escaping the planet and that more infrared of the same frequencies is being reflected back to Earth’s surface. While many factors affect our planet’s climate, there is overwhelming evidence that CO2 is the prime cause of its recent warming.

Climate unknown: How much greenhouse gas to expect

The biggest uncertainty is human… Our current emissions trajectory is close to the worst-case scenario of the Intergovernmental Panel on Climate Change (IPCC). If we continue on this path, CO2 levels could hit 1000 ppmby 2100 – or perhaps even higher.

The second uncertainty is Earth’s response… Currently, rising CO2 levels are driving global warming, but in the past CO2 levels have naturally risen in response to rising temperatures. We do not know why exactly, but the reduced solubility of CO2 in warm water and changes in biological activity have been suggested as reasons. If such mechanisms kick in, even bigger cuts in emissions will be needed to limit warming.

There are also vast quantities of greenhouse gases locked away in permafrost, in peat bogs and undersea methane hydrate deposits. We don’t know how big these stores are. Nor do we know how much permafrost will melt, or how much peat will dry out and decay, or whether the seas will get warm enough to trigger the release of methane – an even more potent greenhouse gas than CO2 – from the hydrates.

Climate known: Other pollutants are cooling the planet

We pump all kinds of substances into the atmosphere. Nitrous oxide and CFCs warm the planet as CO2 does. Black carbon – soot – warms things up overall by soaking up heat, but cools Earth’s surface by shading it. But other pollutants reflect the sun’s heat back into space and so cool things down…

Burning sulphurous fossil fuels has been adding huge amounts of SO2 to the atmosphere. Between the 1940s and 1970s, this pollution was so high that it balanced out warming from CO2. But as western countries limited sulphur emissions to tackle acid rain, the masking effect was lost and global warming resumed.

Sulphur emissions began rising again in 2000, largely as China built more coal-fired power stations. Now China is installing sulphur-scrubbing equipmentin those power stations. If SO2 emissions fall, global warming could accelerate…

Climate unknown: How great our cooling effects are

Pollutants that form minute aerosol droplets in the atmosphere have horrendously complex effects. How much radiation is reflected by sulphur dioxide aerosols varies according to the size of the droplets, their height in the atmosphere, whether it is night or day, what season it is and several other factors…

But if aerosol cooling is larger than generally assumed, the planet will warm more rapidly than predicted as soon as aerosol levels fall.

Climate known: The planet is going to get a lot hotter

Take water. Water vapour is a powerful greenhouse gas. When an atmosphere warms, it holds more of the stuff. As soon as more CO2 enters a watery planet’s atmosphere, its warming effect is rapidly amplified.

This is not the only such “positive feedback” effect. Any warming also leads to the rapid loss of snow cover and sea ice, both of which reflect sunlight back into space. The result is that more heat is absorbed and warming escalates. Longer timescales bring changes in vegetation that also affect heat absorption, and the possibility that land and oceans begin to release CO2 rather than absorb it. Over hundreds or thousands of years, vast ice sheets can melt away, further decreasing the planet’s reflectivity. Barring some unexpected catastrophe such as a megavolcano eruption, then, the planet is going to warm considerably.

Climate unknown: Just how much hotter things will get

The bulk of the evidence still points to a short-term climate sensitivity of around 3 °C, as the IPCC’s models suggest. But while a figure much lower than that is unlikely, there is a significant probability of higher sensitivities (see diagram)…

Climate unknown: How things will change in each region

Even with an average global temperature rise of just 2 °C, there will be some pretty dramatic changes. Which regions are going to turn into tropical paradises? Which into unbearably humid hellholes? Which into deserts? For planning purposes it would be useful to know.

Unfortunately, we don’t. The broad picture is that the tropics will expand and get a bit wetter. The dry zones either side of the tropics will get dryer and move towards the poles. High latitudes will get much warmer and wetter.

When it comes to the finer details, though, there is not much agreement…

Climate known: Sea level is going to rise many metres

Studies of sea level and temperatures over the past million years suggest that each 1°C rise in the global mean temperature eventually leads to a 20-metre rise in sea level.

That makes the effects of a rise of at least 2°C rather alarming. How alarming depends on how quickly the great ice sheets melt in response to warming – and that is another big unknown.

Climate unknown: How quickly sea level will rise

We have little clue how much room we have for manoeuvre. Past melting episodes provide little help. Melting can be rapid: as the last ice age ended, the disappearance of the ice sheet covering North America increased sea level by more than a metre per century at times. It is unclear if Greenland’s ice will melt as rapidly.

To predict exactly how quickly sea level will rise, we would first need to know how much hotter the planet is going to get. As we have seen, we don’t.

Climate unknown: How serious the threat to life is

Many species will have to move to stay within a tolerable temperature range. Animals will also have to change their time of hatching or migration to stay in sync with food sources. Many won’t make it: theoretical studies based on relatively conservative warming scenarios have come up with dire estimates of a third or more terrestrial species going extinct. Real-world studies of the effects of warming so far have backed these conclusions.

Climate known: There will be more floods and droughts

Warm air holds more moisture: about 5 per cent more for each 1°C temperature increase. This means more rain or snow overall, and more intense rain or snowfall on average.This trend is already evident, and is stronger than models predict.

More intense precipitation means more floods…

Although most of the world will get more rainfall on average, dry periods will still occur from time to time. When they do, soils will dry out faster because of the higher temperatures. Once soils dry out, the sun’s heat goes into warming the land rather than evaporating water, triggering or exacerbating heatwaves.

Climate unknown: Will there be more hurricanes?

As the lower atmosphere gets warmer and wetter over the coming decades, there will be more fuel available to power extreme storms. But how often will this fuel ignite? Hurricanes are relatively rare because they form only when conditions are just right. While higher sea-surface temperatures will favour their formation, stronger high-level winds may rip them apart. The result could be fewer hurricanes overall, but with greater strength when they do occur. As the destructive power of hurricanes rises exponentially with increasing wind speed, a few intense storms could wreak more havoc than many weak ones.

At temperate northern latitudes, the news might be better. There winter storms are powered largely by the temperature differences between cold air from the poles and warmer air masses from the tropics. Such storms may become less common as rapid warming in the Arctic reduces the temperature differences.

Climate unknown: If and when tipping points will come

If the Arctic suddenly cooled, sea ice would recover within a few years. If the great ice sheets of Greenland and Antarctica lose enough ice to raise sea level a metre or more, though, it would take thousands of years for snowfall to build up the ice sheets again. The risk is real: we know that the West Antarctic ice sheet has collapsed many times in the past, raising sea levels at least 3 metres.

We can identify many other such dangerous “tipping points“. The Amazon could flip from being rainforest to grassland, just as the Sahara suddenly dried up 8000 years ago. Massive amounts of methane could be released from undersea methane hydrates.

I really like the concluding article. Here it is (most of it anyway) in its sciency glory:

The biggest climate change uncertainty of all

WOULD you jump off a skyscraper? What if someone told you that physicists still don’t fully understand gravity: would you risk it then?

We still have a lot to learn about gravity, but that doesn’t make jumping off a skyscraper a good idea. Similarly, we still have a lot to learn about the climate but that doesn’t make pumping ever more greenhouse gases into the atmosphere a good idea.

Uncertainty is one of the defining features of science. Absolute proof exists only in mathematics. In the real world, it is impossible to prove that scientific theories are right in every circumstance; we can only prove that they are wrong. This provisionality can cause people to lose faith in the conclusions of science, but it shouldn’t. The recent history of science is not one of well-established theories being proven wrong. Rather, it is of theories being gradually refined. Newton’s laws of gravity may have been superseded, but they are still accurate enough to be used for many purposes…

In fact, perhaps the biggest source of uncertainty is not to do with the science at all, or the global climate system, but with us.

Will we burn every last drop of fossil fuel? Or will some amazing technological advance make the switch to renewable energy a no-brainer? Will we keep building cities in places vulnerable to sea-level rise, like Shanghai?

Even politicians who back action to curb global warming are not delivering on their promises. Many of the countries that signed up to the Kyoto protocol have failed to achieve their very modest targets. Meanwhile, some countries in Europe are signing up to more ambitious goals for reducing emissions by 2030, while still commissioning coal-fired power stations.

By the time the need for drastic action becomes blindingly obvious, the best opportunity to curb harmful change will have been squandered. Yet if draconian action is taken today, any success in limiting warming will be greeted with scepticism that drastic measures were ever worthwhile or even necessary. Perhaps the greatest unknown, then, is how to persuade people to act today to help protect their long-term future, not to mention future generations.

One more thing is certain: only science can reveal how our planet can provide a decent home for billions of people without toppling over the precipice.

I love it. Succinct, honest and forthright. 

2 Responses to What We Do and Don’t Know About Climate Change

  1. Pingback: Climate Change Science In the News Again « Science Picks

  2. Pingback: The Prospects for Alternative Energy « Science Picks

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