Climate is everything to life on Earth. Life dances to climate’s tune. Each organism has a narrow band of temperature, water, and nutrient needs, whose availability are determined by climate. When climate changes, organisms move to find conditions they need. Migratory animals do this seasonally. Trees do this over centuries and millennia, seeds blown on the wind to more favorable locations.
Great life extinction episodes correspond to drastic changes to climate. The smallest organisms have always found a survival niche to avoid complete life destruction. But organisms as complex and large as humans typically do not survive. The last Glacial Maximum was the first adverse climate change to involve our species. We survived because we were mobile, were a small population, and could reach refugia in temperate climes.
Humans would do well to understand climate and to plan for our next refugia. We have a small problem, though. Where will we find refugia to accommodate our soon-to-be 10+ billions?
To learn some lessons we need to know, we turn to climate history.
Earth’s Climate Regimes – Climate Change Writ Large
Deep history climate proxies, such as deep sea bed cores, tell us at low resolution that Earth’s climate has undergone several distinct regimes, seemingly forced by the dynamic reconfiguration of continental orientation, itself the result of plate tectonics. The current regime began perhaps 2.5mya.
The Pleistocene – Earth’s Most Recent Climate Regime
At the start of the Pleistocene, the North and South America continents became connected by the final episode of the gradual Central America uplift process, blocking ocean currents between the Atlantic and Pacific equatorial basins. Since the gap closed completely, the Atlantic has experienced slightly less surface elevation and slightly more salinity than the Pacific, due to the reconfiguration of the ocean’s currents.
Recent reports suggest Antarctica was much warmer and had trees growing significantly inland as late as 3mya. But the Pleistocene commenced a global ice age regime, rendering Antarctica the coldest, driest place on earth ever since.
Pleistocene Intra-Regime Climate Change
Our current Pleistocene regime has consisted of periodic glaciation stages. Each such stage is itself comprised of a complex series of warming and cooling cycles. A number of near-term paleo-climate proxies with medium resolution, among them the North Greenland Ice Core Project (NGRIP), inform us of climate change within the current glaciation stage. Resolution is increased by correlating several available proxies.
Relative to the current warm temperature norm, during the last glacial maximum (LGM) the average Greenland temperature at 75°N was ~17K less (17 Kelvin), the temperature at European latitude 45°N was ~8K less, and the sea surface temperature of the equatorial west Pacific dropped ~3K.
The reason for the pseudo-cyclic intra-regime coolings is complex and poorly understood, likely involving many variables and multiple nonlinear feedback processes. Suspected variables include:
- variability of atmospheric greenhouse gas densities, resulting from human activity, volcanism, and methane burps (release of methane hydrates due to seabed and permafrost warming)
- variability of deep ocean currents due to changing temperature and salinity patterns (Earth’s climate pump)
- variability of CO2 absorption by the ocean (CO2 -> H2CO3), due to changes in ocean temperature, acidity, and circulation
- variations in insolation (the sun’s energy output waxes and wanes)
- variability introduced by irregularities of Earth’s orbital mechanics
- variability introduced by under-ocean volcanism, itself influenced by orbital mechanics’ effect on sea level
- variation in cloud cover (there is some evidence that clouds are seeded by cosmic rays, where cloud variability results from the inverse effect of solar activity on cosmic ray intensity)
- variability of ocean albedo due to wind/wave variability
- complex ice sheet stability influences including sea level/temperature and geothermal inputs
- ocean-induced pattern variation in jet streams and arctic winds
- likely even more variables just under the radar of our limited insight.
Our Current Climate System
There is an observed bipolar coupling of glaciation, climate, and oceanographic processes. Complex, interdependent models struggle to deal with system hysteresis. According to Siddall et al. (2008), while global mean temperature may be directly coupled with the Arctic NGRIP records, sea level decrease and glacial expansion may be coupled with Antarctic warming, related to but out of phase with Arctic warming. The models must further explain short-term Heinrich events, up to six observed periods of continental ice sheet instability that preceded some Arctic warming events (or followed some Antarctic warming events) during the last glaciation stage.
The ocean together with land and sea ice mass provides the state memory for the climate system, the oceans being the great temperature sink that drives atmospheric processes. The top two meters of ocean surface has more thermal capacity than the entire atmosphere.
Meridional currents couple the events at both poles; they provide our basic climate pump. Salinity and temperature gradients are the principal factors in establishing deep ocean currents, in combination with mechanical assistance of wind-driven surface currents. There are two known buoyancy states of our thermohaline ocean system: thermal forcing and haline forcing. The oceanic current patterns are very different depending on which buoyancy is dominant.
Flux characteristics of heat and salinity are different; their coupling in the system involves a non-linear process with multiple boundary conditions and equilibrium points. Remarkably, the system appears, from our short-term vantage, to be relatively stable. It is hypothesized that advection via these currents dominates other factors in overcoming local imbalances. But tipping points, thermohaline catastrophes when the system state flips, are hypothesized as having triggered global climate change in the past; such behavior has also been modeled by researchers.
The Holocene (and likely prior interstadials) has occurred under the current thermal forcing regime. This is also the anthropocene, our time on Earth. Thus we are able to scientifically explore and understand how thermal forcing works. It will be our distant successors here who will have first hand knowledge of haline forcing. It should be our highest priority to understand the nature of the current exhibited stability, and to establish/model the conditions that will produce instability. For we likely do not want to live on Earth under its alternate regime.
Via thermohaline circulation under dominant thermal forcing (aka Atlantic meridional overturning circulation or AMOC), the northern hemisphere of the Holocene is kept near its optimum temperature. During an AMOC cycle, surface waters are transported to higher latitudes by wind. During the trip, evaporation slightly increases surface water salt content (density increase), while increasingly cooler air temperature reduces sea surface temperature (density increase). Once the surface density reaches a limit, the cool, salty surface water submerges in large vertical columns in the Greenland and Norwegian Seas, and returns to the Antarctic in a slow deep ocean current, the North Atlantic Deep Water (NADW). One cycle of the water in the pump has been estimated to last 1.6ky.
At some point, the NADW water rises again. While this upwelling process has long resisted explanation, first scientific data suggest the deep currents are forced up by underwater mountainous areas, allowing them to mix with warmer surface waters. This was first observed during testing in Drake’s Passage and may represent a more widespread phenomenon. Such upwellings cause warming and salinity decrease due to dilution, allowing the deep waters to remain on the surface for the wind-aided trip north again.
Yet another deep current of very dense (saline) water forms from water left after the sea ice floes augment themselves by freezing surface water around the Antarctic. This salty water is so dense that it flows under the arriving NADW current. The following graphic (from Wikipedia) illustrates the operation of the pump.
Warming global temperatures during a thermal-forcing regime seems the likely driver of state flip. It takes a lot of fresh water at the surface of the ocean in the northern latitudes to stop thermal-forcing. Fresh water comes from precipitation and ice melt. Both increase with global temperature. When the salinity of arctic waters is sufficiently reduced and arctic temperatures are sufficiently warm, the surface water remains buoyant and the thermohaline system state flips. Haline forcing returns and the Earth’s climate cycles back to cold again.
Under a haline-forcing regime, equatorial surface density increases (increased salinity) due to evaporation, possibly with inhibiting effect on cold water upwelling. With both low latitude upwelling and high latitude sinking inhibited, the northward meridional warm current conveyor belt stops; there is no longer a NADW and Gulf Stream. Replacing the NADW is the deep, dense cold current from Antarctica, but it flows in the opposite direction from the NADW, from south to north.
We can speculate about how the cycle reverts once again to thermal forcing. The ocean becomes stratified during haline forcing, with little vertical motion between layers, inhibiting the oceans’ ability to absorb CO2. This allows atmospheric CO2 to increase from its natural sources, eventually warming the atmosphere and oceans and leading to a flip back thermal forcing.
Interruptions of the AMOC are thought to limit global warming trends, causing the entire Earth to become colder, with the northern hemisphere particularly affected early on in the cooling cycle. There has been some reported evidence that thermal forcing is running out of steam in the current cycle. One report has the NADW flow decreased by 30% and another suggests the number of columns of high-density sinking water in the Greenland Sea has decreased from 12 to 2. These are likely false alarms for the near term, but also potentially harbingers of a dire distant future.
Longer term systematic investigations are under way and they initially suggest there is a lot of annual variation in the thermohaline currents. Not enough data is available yet to identify any long term trends. So far as we know, the conveyor belt circulation is quite stable, although there exist models and historical climate proxies that suggest the pump gets recycled periodically, maintaining our global mean temperature in a band that has afforded habitable climate for most Earth species, even if in a stressing environment.
Climate Change and Us
Perhaps the amount of carbon dioxide we have stored in our atmosphere/ocean system will alter the timeline for a future thermohaline state change and the accompanying new big freeze with its catastrophic effect on the majority of Earth’s human population. If so, we might reason that burning coal and driving cars with monster engines is somehow insurance against such a dismal future time. (No models showing such a mitigation are noted yet:)
A more pressing concern, in terms of centuries rather than millennia, is upon us already. The amount of carbon dioxide in the atmosphere and oceans is at an all time high for the Pleistocene and rising rapidly. This is the result of recent human activity, mainly due to the burning of fossil fuels and biomass. Our immediate future generations will need drastic intervention by us now to prevent very problematic climate change within their lives and the lives of their children. Such inhospitable futures seem increasingly likely to be baked into our atmosphere, irreversible at any acceptable cost.
The only meaningful intervention involves economically convincing humans to leave remaining fossil fuels in the ground, by taxing carbon release at its real cost to us. Our atmosphere can no longer be a free garbage dump.
The lost energy will need to be replaced by immediate addition of clean energy ‘wedges’, nuclear (thorium), wind, geothermal, tidal, solar, and hopefully many more, to replace the dirty energy. Energy conservation will also be needed to close the near term gaps. Current political realities demand that future energy sources compete with coal on a cost basis. This would be acceptable if the cost of coal were its true cost to us, not just the cost of digging and transporting it.
Experts set our necessary goal for the current century, to limit average global warming to an additional 2K. But in spite of our 2016 global climate accord, meeting such a goal would at best require a minor miracle. We have the means, but not yet the will. So new plagues are destined to befall us, such as violent climate extremes leading to drought and desertification; loss of coral reefs and most shellfish due to ocean acidification; global loss of biodiversity.
Our descendants will owe their deprived condition to our present populace’s majority opinion, outright denial of negative possibilities. These are the good folk, true the general human condition of selfish priorities, who do not fix the roof if it isn’t raining, who sell the house and let the next guy handle the problems. But to whom can we sell a failed biosphere? And even the smartest among us, whom we count on to guide our policies, likely rely on climate models that assume too much stability, particularly regarding the AMOC. When the policy under examination can have such a profound effect on our futures, we need more critical challenging of hidden assumptions.
We are unable yet to predict whether our climate course will remain within the Pleistocene normal, a perpetual ice age punctuated by short periods of nice weather like now, the only weather modern humans have ever known. It is in our best interests to make the nice weather, of the last 10K years, last for as long as possible. For we as a species, even with the vast technologies at our disposal, have long lost the capability to adapt to the Pleistocene normal climate. Our future is destined to be unimaginably different, and seemingly not for the better.
To help ensure we make the nice weather last, human ingenuity should be employed to allow us to take control of the climate pump, via some human forcing regime, so that we can all live in the optimum climate that will be necessary to feed our 10+ billion people.
This astronomically growing population is the real back story here. Yet as humans are wont to do, we will pass the buck and allow nature to deal with that, in our unimaginably different future.