Source: Hot Topic – By Gareth Renowden – Analysis published with permission of 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.
Putting it all together, it adds up to the non-uniform pattern of sea ice change we have seen in the last 40 years: increases over the Ross Sea (south of New Zealand) and over the Weddell Sea in the far South Atlantic, where the winds have trended more southerly (colder), and decreases near the Antarctic Peninsula, where the winds have trended northerly (warmer). Other factors in the overall sea ice trend include the melting of ice from the Antarctic ice sheets, putting easily-frozen fresh water into the southern oceans, and changes in ocean surface waves that have affected the break-up and merging of ice floes.
Meanwhile, back in the Arctic, we have a fairly quiescent situation with the sea ice melting away at an accelerating rate, as the ocean surface soaks up sunlight. The differences in what’s happening with sea ice at both poles has a lot to do with the detail of geography, winds, the nature of the ocean circulation, and even El Niño and the ozone hole. What we are seeing from year to year are intermediate steps along the way to that generally warmer world, with less ice all round and “polar amplification” at both ends of the earth. We will get there, if we wait long enough.
So what’s in store for the future? The last IPCC report demonstrated clearly that the amount of global warming we experience depends a lot on how much more CO2 we emit. The two extreme scenarios considered by IPCC were the low-carbon future of scenario “RCP2.6” and the high-carbon future of scenario “RCP8.5”. I call these the blue future and the red future, from the colours used in the IPCC report. Under the blue future, emissions are projected to go to zero by around 2060, then become negative after that (CO2 removal, using technologies we haven’t quite invented yet). That scenario stops the warming before we get to 2°C change, and is the only one considered in the IPCC report to do so.
The red future is “business as usual”, just keep burning the coal and oil like we have the last few years. That results in global change beyond anything seen for probably 50 million years. This is the “crocodiles swimming at the North Pole” scenario.
So, what about that blue future…? The one all the governments signed up to in Copenhagen a few years ago? There is a clear illustration of the situation in the Ministry for the Environment’s “Discussion Document” issued in May as part of the brief and poorly-publicised public consultation round on what our future national emissions targets should be. That document shows that we have a limited budget of CO2 we can emit, since the stuff stays in the atmosphere so long and just builds up. To have a good chance (67%) of staying under 2°C of warming, we have a limit of 2900 Gigatons (2.9 trillion tons) of CO2. The bad news is that we have already used two thirds of the budget, and at current rates it will be all spent within 20 years. So some really significant action is needed if we are serious about reining in climate change.
We have all heard of the 2°C limit, the “safety guardrail” that we don’t want to cross. Yet 2°C is nothing magical, no guarantee of safety. Already we have had nearly 1°C of warming and we know already that floods and heat-waves are more likely than they were 50 years ago. Still, keeping under 2°C of warming may stop the big ice sheets from melting too much and would avoid the really extreme changes that are possible.
Whatever happens with the total warming, things are bound to play out differently around the globe. For instance, we can look at how long it would take to get to 2°C warming in different places, assuming “middle of the road” emissions. A paper in 2011 by Manoj Joshi and co-authors did just that, and found that much of the Arctic will have passed 2°C of warming within the next 10 years. Going by the huge increase in wild fires in Alaska in recent years, the Arctic may have already over-achieved. Farther south the changes are slower, and over New Zealand and the southern oceans, we’ll have to wait until late in the century. Most of the climate change issues for us will come sooner from what happens to our neighbours and trading partners. There are economic, social, and moral issues associated with climate change impacts in other countries that will put pressure on New Zealand, well before the climate turns nasty here.
More importantly than temperature change, rainfall patterns are shifting. It is becoming drier in the subtropics and wetter nearer the poles (and on the Equator). At the latitudes of Australia and northern New Zealand, we are likely to see a lot of drying over coming decades. In the Northern Hemisphere, a very worrying sign is the drying out of the Mediterranean region, from North Africa to the Middle East to southern Europe. This is already a place with lots of issues – political unrest, terrorism, war, economic crises, huge flows of refugees… beyond its direct effects, climate change is an aggravator of all these things. Organisations like the World Economic Forum and the World Bank, even the Pentagon, recognise this and list climate change as an immediate threat to social order worldwide
And let’s not forget sea level rise – another big worry, largely because it is so inexorable, and so much of the global population lives close to sea level. Once perturbed, the ocean circulation and the big ice sheets take a long time to respond, so we are in for a long period of sea level rise regardless of the emissions future. Going back to the blue and red futures, the models show sea level rising steadily through this century and beyond under both scenarios. Even on the zero-carbon track, we are set for at least 1m of further sea level rise, over centuries. And as the geological record says, we will see 8, 10, even up to 20m or more if we carry on as we are going now.
So, what are the consequences, the impacts? Key ones that concern me are:
- 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.
This is what we face. In fact, this is what we are starting to experience already. So how do we get on top of it? Can we get on top of it?
Yes! There are many technologies and ideas on the shelf that we can use right now. Renewable energy is an obvious one (go China!). For all their coal-fired power stations, China is leading the world on solar panels and wind power installation and technology. New Zealand can ride on the coat-tails of the Chinese and go to 100% renewable energy – despite a high base, we can go a lot further here. And if we wished, New Zealand could be a world leader on renewable technology – are we content with being a “fast follower”?
Same story with electric vehicles (go Tesla!). The transport sector a big one in New Zealand and transport emissions have grown rapidly in the last two decades. We love our cars – which is fine, if they aren’t burning fossil carbon. Let’s see moves to bring electric vehicles in to the country in much greater numbers, while at the same boosting public transport and making the most of renewable power sources. That could cut our emissions significantly in just a few years.
In the agriculture sector, continued intensification of dairy farming is exactly the wrong direction to be going. It is just not sustainable, especially in dry regions like Canterbury, in terms of water quality, water availability, and greenhouse gas emissions. A much better approach in the short term would be intensified afforestation, which would at least buy us some time to do the research on ruminant emissions.
The solutions that already exist can work in New Zealand and can be applied world-wide. We need all of the above, and we need to find new and better approaches every day. As put so eloquently by the Pope just last month, there are moral dimensions, questions of equity, of love for one another, that must take centre stage. Narrow economic considerations must be secondary, as no known economic modelling framework can cope with the true realities of climate change.
What is lacking across the board is political will. Governments set the scene for a country’s economic and social activity. All countries, including New Zealand, need to tackle climate change head-on through legislation, through incentivisation of desirable investments and behaviours, through economic instruments that encourage research and innovation in the sectors that we need to boost.
The recent ruling by the Dutch courts that their government is harming the population if they do not adopt stringent emissions reductions (25% reduction in 5 years) is exactly right. Governments the world over are indeed putting their citizens more at risk every day by not dealing effectively with climate change. Where is the sense of urgency? Sure there are many worries and concerns in the world, but unmitigated climate change exacerbates almost all our short-term concerns, and ultimately trumps everything. Do we really want to put billions of lives at risk through hunger, thirst, disease, dislocation and conflict, in order to appease the corporate sector and win the next election?
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!