Cold winter weather on a warming planet
Ah, ‘tis the season of our discontent. It’s been cold where I live. Anyone who switched on the TV during January, and tuned to any weather report would have gained the impression that the next ice age was upon us. South of the Canadian border, weather reports waxed hysterical. It seemed to be snowing and freezing everywhere. And yet our planet continues to warm.
Global pattern of temperature for January 2014. There are several large record warmest areas across the globe, but no record coldest. Map courtesy NOAA National Climate Data Center.
That paradox is explained partly by the distinction between climate and weather – between the long-term condition and trend and the short-term variability that keeps so many meteorologists employed because forecasting weather is so deliciously hard to do. It is also partly explained by our preoccupation with what is happening outside our own front door, and our difficulty, in the midst of a blizzard, to take a wider view. NOAA’s National Climate Data Center has just released its report for January. January 2014, globally, was the fourth warmest January on record, and even in the United States, January 2014 was just 0.1oF (0.056oC) colder than the long-term average for that country. The colder than usual, snowier weather in the east was counteracted by the warm dry weather in the west, while the drought in the US south-west worsened. Further, even here in eastern Canada, while temperatures were at times a lot colder than we have experienced in recent years, they were not a lot colder than average for the last 30 years (the duration over which climate, rather than weather, should be measured). As I noted previously, the weather this winter in eastern North America can be best understood as a consequence of climate change – the warming Arctic means that the temperature differential between Artic and Temperate regions is lessened, and as a consequence the jet stream is slowing, and wobbling. Indeed, the cold weather this winter has been arriving in week-long bursts followed by only somewhat shorted periods of relatively much warmer weather – evidence of that wobbling jet stream and the resulting shifts in the polar vortex. Set thoughts of a new Ice Age aside, and know that Spring is coming.
Of course, if we understand the science, we know that the Earth must be continuing to grow warmer. The concentration of CO2 in the atmosphere continues to increase (atop Mauna Loa it topped 397.8ppm in January) which means that the extent of insulation surrounding our planet also continues to increase. Each year, progressively more of the sunlight-derived heat is retained and the planet warms. Much of that warming appears in the form of a warming climate; much of it is embedded in the warming oceans. Remember that water has an enormous capacity (compared to air) to take up heat – that is, it takes lots of heat to warm water by a degree or so – and our oceans have been warming measurably. NOAA reports January’s global sea surface temperature as the 7th warmest on record and the warmest since 2010.
Sea of Complexity – The Oceans and Global Warming
1. The oceans and the warming hiatus
This brings me to some interesting recent science on what is happening, and what may be happening soon as our oceans warm. On January 16th, Nature published an open access article by Jeff Tollefson that reviews the likely reasons for what has been called the ‘global warming hiatus’, an apparent slow-down in rate of warming of the climate that began in 1997. It is well worth reading. This hiatus has been seized upon by various denialists as evidence that global warming is not happening, and at first the climate science community did not have an explanation beyond natural variability in climate – that explanation gets less satisfying as the slowdown in warming continues, and the science community has been hard at work investigating what may be going on to have the continually added heat not show up in progressively warmer climate. (Previously, I had mentioned a study by Steven Sherwood and colleagues, also published in Nature, that had investigated details of the complicated effects of low level cloud on climate, another example of the way in which the scientists are continually refining their understanding of how the planet is responding to the growing atmospheric concentrations of greenhouse gases.) Tollefson eloquently summarizes the advances in understanding related to the hiatus in warming, proving once again that while science never has all the answers, science is an effective process for building knowledge and gaining understanding – something the denialists don’t seem to understand. Tollefson’s article reviews various hypotheses to explain the apparent slowdown (it’s still possible, even after 16 years, that it is an anomaly due to natural variability, so I tend to use ‘apparent’ when referring to this hiatus).
Tollefson focuses in on the behavior of that enormous heat reservoir, the Pacific Ocean, and specifically on the el Niño – la Niña cycle and the associated Pacific Decadal Oscillation. This oscillation is an approximately 15 to 30-year cycle in which the tropical Pacific alternates between predominantly el Niño conditions with only light westerly, or even easterly winds allowing warm surface waters to accumulate in the eastern Pacific, and predominantly la Niña periods in which stronger westerlies transport surface waters towards the western Pacific, allowing colder, deeper waters to rise to the surface of the eastern Pacific. These upwellings keep the surface waters of the Pacific cooler, enabling a greater uptake of heat from the atmosphere. Putting it simply, the cold phase of the Pacific Decadal Oscillation is a time when heat can be transferred from atmosphere to ocean more rapidly than during the warm phase, and we have been in a cold phase since about 1997.
Chart showing the approximately 15 to 30-year alternation between periods when the Pacific Decadal Oscillation is in a warm phase (red), and in a cold phase (blue), and the pattern of change in global temperature from 1920 to the present. Chart © Univ. Washington/IPCC.
One thing is very clear. If the Pacific Decadal Oscillation is having this effect, it will eventually switch to its warm phase and atmospheric warming will spike upwards again. That could happen any time now or perhaps not for another decade. As Nature’s editorial that week stated,
“From a policy perspective, little has changed. The range of potential impacts projected by climate models warrants much more aggressive action than has been initiated so far.”
2. Changing Arctic albedo and warming
If the Pacific has been remaining cooler than expected, the Arctic has continued to warm. As it warms, the sea ice melts and white reflective surfaces are replaced by dark, open water. This change in brightness (or albedo) has a direct effect on the rate at which sunlight is absorbed (and converted to heat). On February 18th, an article by Kristina Pistone and colleagues at Scripps Institution of Oceanography, La Jolla CA, was published on-line by Proceedings of the National Academy of Sciences USA. In it, they report their use of satellite data over the past 30 years to document the change in albedo, and to determine the increase in rate of warming that has resulted. Their data are surprising. In their own words,
“the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, corresponding to an additional 6.4 ± 0.9 W/m2 of solar energy input into the Arctic Ocean region since 1979. Averaged over the globe, this albedo decrease corresponds to a forcing that is 25% as large as that due to the change in CO2 during this period, considerably larger than expectations from models and other less direct recent estimates.”
What that last sentence means is that the change in albedo due to melting back of Arctic sea ice has had an effect on warming since 1979 which is fully ¼ that of all the CO2 emissions over that same period of time. The melting of Arctic sea ice provides a positive feedback for climate warming, because the more the ice melts, the more rapidly the ocean warms, escalating the rate of melting. That the melting of Arctic ice has contributed as much to warming as it has is definitely surprising, and their study serves to reinforce past expressions of concern for just how rapidly the Arctic is changing. Things are happening quickly up there, and we have far less understanding of how the Arctic system functions than we do such places as the tropical Pacific. We’ve got a tiger by the tail, and it’s taking us to a destination that we do not know much about.
A tiger by the tail. Image © rm73/DeviantArt.com
3. Slowdown in ocean circulation?
To emphasize just how much our warming of the planet is putting us into dangerous places, I’ll mention just three other studies. The first concerns the formation of North Atlantic Deep Water (NADW). The circulation of the world’s oceans is a complicated set of interrelated processes that produce a global-scale pattern of currents sometimes referred to as the Ocean Conveyor (an animation is here).
The Ocean Conveyor. Water circulates slowly throughout the world’s oceans on a path that brings oxygenated water to the deep ocean and returns deep water to the surface.
Chart courtesy NASA JPL.
The formation of NADW is a crucial engine in maintaining this flow. As surface waters move north in the Atlantic as the Gulf Stream, they cool, thereby transporting heat from the tropical to the temperate atmosphere. Cooler water is more dense, and so this cooled surface water sinks below the water flowing out of the Arctic, taking oxygen to the deep waters of the Atlantic basin. Upwelling regions in the Indian and Pacific oceans restore bottom water to the surface. Formation of NADW was the focus of a short news item by Richard Kerr in Science on 21st February. He reported on an article posted on-line in Science this past week. In that article, Eirik Galaasen, a paleoceanographer at University of Bergen, Norway, described results of analyses of microfossils in sediments deposited south of Greenland about 100,000 years ago. Variations in the carbon isotopic composition of the skeletons of minute foraminifera track changes in the ratio of carbon 13 to carbon 12 (13C/12C) in the waters in which they live, and the NADW, having been at the surface relatively recently is typically high in Carbon 13. By analyzing changes in the 13C/12C ratio of fossil forams in cores through the sediments, Galaasen was able to detect changes in the rate of flow of NADW past the site. During the relatively warm interglacial period between 130,000 and 115,000 years ago, he detected three occasions when the rate of delivery of NADW slowed, stopped or even reversed. Two of these were brief periods lasting about 100 years, but the third consisted of multiple episodes over 2500 years.
When water is warmer, it is less dense, and so at times when the North Atlantic environment is warm, surface waters travelling north may not become sufficiently cooled to sink to become NADW. The result would be a slow-down of the entire ocean conveyor, and a reduction in delivery of oxygen to the deep ocean. Melting of land glaciers would add fresh water, potentially lowering salinity and thereby adding to the tendency of surface waters to remain floating. Galaasen’s results confirm that in the last interglacial, when air temperatures were not notably warmer than those today, the ocean conveyor slowed down on several occasions.
The consequences of a slowdown in the ocean conveyor are several. First, it slows the transport of heat from tropics to the temperate zone, and could be expected to substantially cool climate in Europe. Second it deprives the deep ocean of oxygen, potentially leading to anoxia and loss of deep-water life. Third, the slowing down of the Gulf Stream, could increase sea levels on the North American Atlantic coast by up to a meter because water would ‘pile up’ on the western side of the Atlantic. And finally, fourth, the slowdown in production of NADW would also slow the removal of CO2 from the atmosphere by dissolving in the ocean, exacerbating the warming due to greenhouse gas emissions.
Of course, none of these effects are sufficiently concerning that we should consider altering our behavior and reduce greenhouse gas emissions before they occur. No, indeed, we must protect continued economic growth at all costs. Joel Pett’s cartoon has captured the prevailing attitude. After all, the scientists could all be wrong. Yet Galaasen has showed us that it happened in the past. It just might start happening again.
4. Climate change and Arctic fishes
What about the organisms that use the Arctic? They are experiencing a rapid change in climate. How is it affecting them? Jørgen Christiansen, from the Arctic University of Norway, in Tromsø, and two colleagues (from USA and from Russia) recently published an opinion piece in Global Climate Change. It’s another open access article. In it they discuss a quite indirect effect of Arctic warming on Arctic fauna. They make the point that with warming of the Arctic, commercial fisheries are venturing ever further into the Arctic Ocean, following the fishery populations that are also moving in, following their food species (each species is tending to move with water of the same temperature, and as the Arctic warms everybody moves north). This shift means that even without specifically targeting the truly Arctic species that have long been inaccessible under the sea ice, these commercial fisheries will inevitably begin impacting those previously unfished species.
Now this could be seen as a good thing, an opportunity to fish a new set of species, an opportunity to add new fishery resources in a world that has been losing fishery resources due to overfishing. But Christiansen and colleagues see some real reasons for concern. The Arctic Ocean and adjacent seas include about 600 species of potential fishery species. Of these, 63 are truly Arctic fishes. These 63 species of until now largely unfished species suffer from what they refer to as three shortfalls: a Linnean shortfall, a Wallacean shortfall and a Hutchinsonian shortfall (only biologists will appreciate the esoteric choice of names). Basically, they see these species as inadequately known biologically (the taxonomy is confused and species are not clearly defined), inadequately known in terms of their geographic distributions, and inadequately known in terms of the biology including such details as demography that would be essential information if trying to manage a fishery for them. Christiansen and colleagues express concern that exploitation of these Arctic fishes will proceed, well in advance of the science needed to fill these severe gaps in knowledge, and we will likely overfish them before we even get around to putting fishing regulations properly in place. They call for a concerted effort to learn the biology of these species, and the ecology of the system they inhabit, combined with an emphasis on green fishing techniques – such as avoiding use of bottom trawling given that the community is primarily demersal and territorial. They also argue for a conservative, precautionary approach to fishing, using effective management regimes such as ones focused on total catch, rather than total landings (which ignore all those fish taken as bycatch and dumped at sea), and strong integration of management among the countries that each hold jurisdiction over part of this ocean. Their arguments are sensible, timely, based on science rather than emotion, and form a logical way to begin to use this new source of fish. They also raise the moral issue – these fish support native peoples who must be assured of continued access.
Most waters of the Arctic Ocean lie within the EEZ of one of the Arctic countries. Retreat of sea ice is making larger portions of the Arctic accessible to fishing, and warming temperatures are causing species to shift northwards. Map © Pew Charitable Trusts
I’d love to believe that their arguments will be well received by countries like Canada, which has a vast EEZ in this ocean. But I have the horrible feeling that, as usual, such arguments will fall on deaf ears as we chase the almighty dollar. That has been the history of all new fisheries in the past: we find the fish, we fish them heavily, we finally get around to attempting to manage them after we have already hit them pretty hard, and often times the management never catches up to the exploitation. It’s sort of a fisheries equivalent to mining tar sands before bothering too much about how to reduce the pollution that causes, or remediate the land which is despoiled and the water which is contaminated in the process. Homo sapiens? Wise about extracting value from nature. Not so wise about doing so in a sustainable way.
5. Skipjack tuna responses to climate change
As a final story for today, I have picked another recent article in Global Environmental Change. This one is by Sibylle Dueri of IRD (Institut de Recherche pour le Développement) in Sète, France, and two colleagues, and concerns impacts of global climate change on skipjack tuna fisheries. I picked this one because the skipjack is a tropical species, and this report neatly makes the point that climate change impacts on the oceans are many, varied, and have complex ramifications for the species that live there.
Skipjack tuna, a smaller species, are the most important tropical tuna fishery species world-wide, primarily for the canning industry and local consumption. Image © M Taquet/IRD
Like any open-ocean species, the skipjack is directly sensitive, metabolically, to water temperature, and must also respond to changes in its food supply caused by changes in regional water temperature. At present, the largest population and the major fishery for skipjack is in the equatorial warm pool of the western Pacific Ocean which yields about 2.5 million tonnes per year, 65% of the global catch. A further 20% of the catch comes from the Indian Ocean, and the final 15% is split between the Atlantic and the eastern Pacific. At present, the west Pacific skipjack move back and forth from the western to the central Pacific in response to changing patterns of water temperature driven by the el Niño – la Niña variation.
Dueri and colleagues used a modeling approach to project temperature and oxygen content across depth to 500 m, and across the tropical/subtropical global ocean. They model response of skipjack to temperature and oxygen directly, and response to changes in distribution of their prey. Their modeling used four IPCC greenhouse gas scenarios, and extended to 2100. They caution that modeling on these spatial scales and over this length of time means that results must be accepted with some caution.
Still, the results are revealing. They show that the currently preferred western Pacific and Indian Ocean locations are going to become progressively less preferred as water warms, although subtropical nearby regions will become more preferred and may provide suitable refuges to which the fish will move. They also show that skipjack populations are likely to increase slightly to 2050, and then decline drastically, dragging the fishery down with them. Specifically, between 2000 and 2100 the model projects a 40% decrease in total adult biomass from 10.4 Mt (million tonnes) to 6.3 Mt. In the Indian Ocean, the computed adult biomass decreases from 3.4 to 2.8 Mt (-18%), in the Pacific Ocean from 5.9 to 3 Mt (-48%), and in the Atlantic Ocean from 1.1 to 0.5 Mt (-57%).
Figure 6 from Dueri et al. 2014. Maps showing the progressive expansion of tropical waters no longer suitable for skipjack tuna due to climate change under four IPCC scenarios. RCP8.5 is pretty much the high emissions path we are currently on, RCP2.6 is a path including an extremely aggressive program to curtail greenhouse gas emissions, and RCP6.0 and RCP4.5 are intermediate between these extremes. All but RCP2.6 result in major disruption to the current distribution of skipjack tuna in the western Pacific and the Indian Ocean. Figure © Global Change Biology.
These declining abundances have importance for this currently well-managed fishery species. Catches are going to have to fall also. An additional difficult is that the major fishery in the western Pacific has been successfully managed primarily because most of these waters lie within the large EEZs of numberous small island nations that have learned to work collaboratively to manage the fishery together, including coping with the major shifts in distribution due to the el Niño fluctuation. With the skipjack moving out of the tropical warm pool, they also move into predominantly international waters where people have had much greater challenges to manage fisheries sustainably. Time will tell how well the skipjack do, and how well the fishery is managed in the future.
If there is one simple take-home message for those of us who do not usually fish for skipjack tuna, it is that climate change impacts even the open ocean, and its impacts are both subtle and profound on many species that live there. Just one more argument for becoming more wise regarding the need to rein in emissions of greenhouse gases.
Image © Francesco Marino/FreeDigitalPhotos.net