The surprisingly strong El Niño of 1982-83 showed us what climate change could be like: global, devastating and stealthy.
During 1982, oceanographers didn’t recognize the developing El Niño, but since then an extensive observational and theoretical framework has been built, and it is now possible to predict El Niño events several months in advance.
The success of that El Niño research suggests that we may also have the capabilities to predict climate change, or at least understand the risks associated with it.
Of course, one lesson of 1982 is that we may think we understand climate change when, in fact, we are clueless. But, climate-change research is a priority in many countries, reason enough to be optimistic about our understanding.
Climate has been changing since the beginning of time, and life adapts by evolution, migration and, recently, technology.
There is nothing to fear as long as change happens slowly relative to other changes within human societies. It takes maybe a decade to build and grow a stable business, two decades to raise children. Within just three decades, computers and globalization have transformed the way we work.
So, it is safe to assume that any dramatic climate change that happens within 30 years is threatening to individuals as well as societies.
What could trigger a change in such a short time? Most scientists agree that variations in vegetation, glaciers or the sun’s activity are unlikely to be relevant to short-term change, although the first two serve as good indicators for detecting climate change. The atmosphere, on the other hand, can undergo violent changes within days. Since air is rather thin and can’t store much heat, a good way to look at the atmosphere is that it just picks up heat stored in the oceans and moves it further toward the poles.
This leaves the ocean as the prime agent providing short-term variability to the climate system, because of its large mass and thermal inertia. This doesn’t mean the ocean is the only important component of the climate system, but it does mean that we have to understand the ocean to understand and predict possible climate changes in the next several decades.
The ocean currents are largely driven by wind and confined to the upper 2,000 feet of the oceans. Since the 1970s, it became increasingly clear that they are not sluggish and unidirectional, but filled with whirls and eddies.
Because the eddies are strong compared to the mean flow, it took four decades of long and careful observations before it became possible to quantify the main flows.
A good example to illustrate the magnitude of the ocean currents is the Gulf Stream (not the strongest, but the best observed current), which off Cape Hatteras transports approximately 50 million cubic meters of water per second – roughly the mass equivalent of the entire North American population flushed through a pipe in one second!
Much of this water comes from the Gulf of Mexico, and it is obvious that so much warm water will have a pleasant warming effect for land in the vicinity of the Gulf Stream. Since atmospheric circulation is remarkably similar along the same latitudes or their opposites in the Southern Hemisphere, the Gulf Stream is matched by the Kuroshio along the coast of Japan, the Agulhas Current along Mozambique and South Africa and the East Australia Current. Each of these poleward-flowing currents is matched by interior flows toward the equator. The key to understanding and predicting climate change is finding out if and under what circumstances these currents can change significantly over the next couple of decades – or whether they have changed already.
Ice cores from Greenland and Antarctica, tree rings and sediment cores from the ocean bottom have recorded climate changes over tens of thousands of years (called the “paleo-record”).
Some of these records suggest that the Gulf Stream could be weakened by massive pulses of fresh water (caused in the distant past by a large North American lake emptying into the ocean). These changes happened within a few decades, a fast-enough time frame to be of concern to us, but they also happened during the end of the last ice age, a time not very similar to ours.
However, today we are increasing the amount of greenhouse gases in the atmosphere, and the concern is that this not only leads to a slow, general warming of atmosphere and oceans (there are now only a few scientists left who doubt that this is already happening); but that it could also trigger fast changes in ocean circulation (within decades) with dire consequences for countries whose current climates rely on the heat and moisture from warm currents.
(To keep things in perspective, it should be noted that relatively small annual changes in the Indian monsoon patterns that are due to natural variability already cost more lives than a cooling of Europe ever could.)
It is worth repeating that societies can adjust to many small incremental changes (e.g., building higher dikes, buying winter boots) but not necessarily to one big change.
Thus, slow greenhouse warming may be critical only if it triggers a fast climate change – like a sudden weakening of currents like the Gulf Stream. The paleo-record, theory and computer simulations suggest that such a weakening is indeed possible for the Gulf Stream. This is because unlike most other currents, a third of the Gulf Stream is not driven directly by the wind, but relies on cold, salty water sinking in the Arctic Seas to about 7,000 feet below the surface, flowing southwest and then returning near the surface as warm water flowing past England and Norway on its way back to the Arctic.
Some scientists reason that as the Earth becomes warmer there will be more rain, and the water in the Arctic Seas could become too light to sink. This would reduce the strength of the Gulf Stream, the Arctic Seas would cool further, leading to more sea ice. That ice would reflect more sunlight than the ocean, which would lead to an even stronger cooling, and eventually Paris, say, would be covered under ice. While every single part in this chain of reasoning is sensible, the sum of the uncertainties associated with each part makes the final outcome – Paris under ice – very uncertain.
A closer examination of the problem shows that understanding this density circulation needs to take into account processes as diverse as molecular processes of seawater, details of sea ice formation, the shape of the sea floor and the tides.
Currently the world’s most sophisticated computer climate models are used to study how sensitive this circulation is to greenhouse warming. Most models do indeed show a weakening of this circulation, but they do not show a cooling over Europe (www.cgd.ucar.edu/oce/pubs/03pubs_ files/Holland-ClimDyn.pdf). If we can trust these models, increase in greenhouse gases may only lead to slow general warming with a slow shift in rainfall patterns.
In spite of all the previous and ongoing research, large uncertainties remain.
The available observational record is barely long enough to give a reliable estimate of the mean strength of ocean currents, so it will be close to impossible in the foreseeable future to reliably observe or predict climate change directly. Discussion about possible recent warming of the oceans shows that even 50 years of observations are barely sufficient to detect the warming that we would expect from a rise in greenhouse gases.
Thus, one responsibility to our children is to preserve, if not extend, our observational network to monitor the oceans. Political cycles as well as funding cycles for scientists are much shorter than a decade, so the components of a permanent observation system are always in danger of being abandoned. However, if we want our children to make informed decisions about their environment, then the maintenance of an uninterrupted monitoring system is of utmost importance.
In our current situation, we have to rely on computer simulations to tell us whether we can expect climate changes in the near future, and on politicians to assess the risks involved. Thus, it appears that today’s citizen not only has to trust politicians to make the right decisions (a problem which, like climate change, has been around for ages), but also rely on computer simulations to guide the those decisions. It is up to the reader to decide which of the two is more unsettling.
Markus Jochum is a scientist at the National Center for Atmospheric Research, Boulder. The opinions expressed are Jochum’s and not necessarily shared by other NCAR personnel.
