Hot-Topic.co.nz Headline: A tale of two hemispheres At the end of June, Professor Jim Renwick of Victoria University gave his inaugural lecture. As you might expect of a climate scientist, it concerns what we know about the climate system and where we’re heading. He pulls no punches. Jim has been kind enough to put together a text version of the lecture for Hot Topic: it follows. You can watch the full lecture, with accompanying slides, on the video embedded at the end of the post. We live in a golden age of earth observation. With a few clicks of a mouse on a web browser, any of us can see the state of the global ocean surface, the current condition of the Greenland ice sheet, how much rain is falling in the tropics today, and on and on. Plus, the International Space Station (ISS), and a series of satellites such as MODIS give us wonderful images of our home planet. The climate science community can tell, with unprecedented coverage and timeliness, just what is going on in the climate system. It is a great time to be a climate researcher, but also a worrying time, in both cases because we can see exactly what is changing. One thing the ISS pictures emphasise is just how thin the atmosphere is, a thin blue layer between the solid earth and the blackness of space. Not only is this life-supporting envelope very thin, some of the key gases in the atmosphere are there in only trace amounts, so we can change the properties of the atmosphere easily, by targeting the right gases. The discovery of the ozone hole 30 years ago brought this home with a bang. And we’ve found that build-up of carbon dioxide (CO2) in the atmosphere can have a profound effect on the climate system, right down to the bottom of the oceans. Carbon dioxide is important because it’s a crucial control on the surface temperature of the earth. It is very good at absorbing heat (infrared radiation) welling up from the earth, then re-radiating both up and down, in the process warming the earth’s surface. The effect is very like a blanket put on a bed – what’s under the blanket warms up. More CO2 is like putting another blanket on the bed and less is like taking away a blanket. No CO2 and the earth freezes – temperatures like we had in the South Island in late June would be the norm everywhere, all the time. While there are several other “greenhouse gases”, carbon dioxide is the most important since it stays in the atmosphere so long, hundreds to thousands of years. Since direct atmospheric measurements began in the late 1950s, CO2 concentrations have gone from 315 ppm to about 400ppm (0.04%) now. Concentrations of CO2 are rising steadily, but the numbers hardly sound “dangerous”. But one thing to realise is that many natural changes take place over thousands to millions years. So instead of human time scale of the last 60 years, we must look on the planetary time scale… Luckily, ice cores store bubbles of ancient air that can tell us what CO2 concentrations were, far back in time. If we join the ice core record up with the observations from Hawaii, we get a very different picture – and now it does look alarming! CO2 in the atmosphere has increased blindingly fast, by planetary standards. We have really put a lot of it up there in a handful of decades. For many thousands of years before the present, back to the beginnings of agriculture and modern civilisation, CO2 concentrations have been fairly steady, between 260 and 280ppm. Suddenly (in geological terms) they are 40% higher at around 400ppm. So, how far back do we have to go to find the last time CO2 was this high? The answer is about 3 million years. We are making changes in decades that left to its own devices, the earth system might take hundreds of thousands of years to effect. Back then, in the “mid-Pliocene warm period”, temperatures were around 2-3°C higher than present, but sea levels were around 20m higher. That much sea level rise takes time, but it will happen again if we allow CO2 levels to stay up there. How do we know about what was in the atmosphere 3 million years ago? From the chemistry of rocks – no ice core goes back far enough so we must look at the chemical composition of the rocks laid down then, as they carry the fingerprint of the chemical composition of the atmosphere. That is, we can read it in the earth itself. The flip side of this is that sediments being formed today will tell the story of today’s big CO2 spike. In other words, our actions today are being written into the crust of the earth and will be visible for millions of years to come, if there are any able to read it. But what about what happens in our lifetimes, what’s happening now? The geological record is no help there – we must just experience it as we go. Global mean temperatures are going up, just what we’d expect from increased carbon dioxide levels. Things are simple at that level: more CO2 = higher temperatures. But climates vary strongly around the world, and so does climate change, as a result of geography, latitude, land mass size and so on. For example, surface temperatures are changing at wildly different rates in different places. Over the last 60 years or so, the global average warming has been around 0.6°C. The Arctic has seen much more and the southern oceans and Antarctica much less. This brings up the issue of “Polar amplification”, the observation from the geological and paleoclimate record that both poles always warm or cool about twice as much as the global average. This is visible for the cooling at the last glacial maximum, and for the warming during the mid-Pliocene warm period. We know from the past that this always happens, but we are now learning that the two poles do not respond at the same rate. The Arctic, with its thin layer of sea ice and snow, can warm quickly. The Antarctic, with its massive ice sheets and turbulent circumpolar ocean, warms only very slowly, over centuries. Where this difference between the hemispheres is really visible is in sea ice. In the Arctic, sea ice is disappearing at a rapid rate, while it is increasing (slowly) around the Antarctic, especially over the last 5-10 years. How can Antarctic sea ice extent be increasing, in a warming world? The number one reason is geography. The Northern Hemisphere features ocean at the pole and lots of land in the middle latitudes. At the pole, there is only a thin cover of sea ice, a few metres thick. The Southern Hemisphere is almost the exact opposite, a big continent over the pole and almost no land in the middle latitudes. At the pole, vast ice sheets have built up, thousands of metres thick. Following from that, the winds in both hemispheres are quite different in form too. In the Northern Hemisphere, the winds are strong over the oceans but not so much over land, and over the Arctic, the winds are very light on average. So the Arctic Ocean is mostly quiescent, with weak currents and little vertical mixing. Any extra sunlight absorbed when Arctic sea ice melts stays in the upper ocean, warming the surface quickly and promoting more melting. In the Southern Hemisphere, the westerlies are very strong and unimpeded over the southern oceans, the most turbulent region of ocean in the world. Here, water is mixed down several hundred metres, so the heating from absorbed sunlight gets drawn down to depth quickly, leaving the surface temperature mostly unchanged while waters warm at depth. So that “ice albedo feedback” works less well for the sea ice over the southern oceans. The Antarctic sea ice grows out around the edge of a continent, over very turbulent waters, with strong winds and storms above. It seems almost miraculous that it manages to grow to such an extent, so regularly every year. The westerlies, their strength and position, are very important for determining how the sea ice grows. And those westerlies have been strengthening and contracting farther south over the last few decades. The strength of the westerly winds and the turbulent storm tracks that accompany the strongest winds, are controlled by the north-south temperature gradient, the difference in temperature between the tropics and the poles. A bigger difference means stronger winds. How that is changing is a key to understanding what’s going on with Southern Hemisphere winds, and with the sea ice. There are several things that affect the north-south gradient…
- The ozone hole (surprisingly!) – removing ozone from the atmosphere over Antarctica cools the polar region (since ozone absorbs sunlight), so increases the north-south gradient.
- CO2 (GHG) increase – away from the earth’s surface, greenhouse warming increases temperatures faster in the tropics than at high latitudes, so also increases the gradient.
- El Niño/La Niña (ENSO) – an El Niño event warms the tropics and increases the north-south gradient, while a La Niña does the opposite, for a few months. Crucially though, the ENSO cycle puts kinks in the westerly flow, making it more southwesterly in some places and more northwesterly in others.
- Drought – recent droughts and heat waves in North America and Russia have led to partial crop failures and price spikes for corn, wheat and other staples. Future droughts have obvious impacts on food security and water availability for large fractions of the global community.
- Flood – as we have seen three times in New Zealand in the past two months. Warmer air holds more water, and the near-one degree of warming so far globally has put about 5% more water vapour in the air compared to the 1950s. So it’s fair to say that some of the rain that fell on Dunedin, Kāpiti and Whanganui was there as a result of the warming we have already had. Further warming just means more moisture and an ever-greater chance of heavy rain.
- Coastal inundation – higher sea levels, even small-sounding amounts like 30cm or so, lead to dramatic increases in the chance of inundation events when there are big swells and strong winds.
- Health issues – as the globe becomes more “tropical”, tropical pests and diseases can spread farther. Malaria, dengue fever and other diseases are broadening their range right now. The same goes for plant and animal pests. And the health dangers of heat waves are only too apparent, as we have seen in India and Pakistan lately.
- Fire – the incidence of wild fires, and the length of the fire season, is increasing almost everywhere. Siberia and Alaska are now experiencing major forest fires regularly, events that were almost unknown 30 or 40 years ago.
As a global community, we have squandered the last 25 years. The Paris meeting in December (COP21) is a critical opportunity to really get good things happening on a global scale, and on the home front. Greenpeace’s protest at Parliament in June was spot-on – what we really need is climate action, now!–]]>