Avoiding A Frozen Earth
Forty million years ago, Earth began slipping from a “hothouse” climate to an “icehouse” climate. Currently the planet is in a brief warm interlude know as an interglacial—a period of retreating ice sheets and shrinking glaciers. As the word interglacial suggests, our current comfortable climate is not permanent, but merely a pause between frigid ice age conditions. Though climate alarmists and media talking heads continue to natter on about uncontrollable rising temperatures a more devastating climate change would be a descent into an ice age so cold and so deep that the entire globe freezes over—it has happened before. A new scientific paper reveals what researchers say is a feedback mechanism that acts as a natural thermostat and keeps Earth from cooling to the point of uninhabitability.
Most of the news regarding climate change is focused on warming and the mechanisms that regulate heat retention. The chosen culprit in the global warming brouhaha is the trace gas carbon dioxide, a known greenhouse gas. What is seldom mentioned is that CO2 does not have a linear effect on heat rate in the stratosphere or elsewhere. It has a much greater impact, proportionately, at low relative concentrations and a decreasingly weaker impact as atmospheric concentrations grow.
One of the upshots of this nonlinearity is that dropping CO2 should yield accelerated cooling, perhaps to the point of runaway global cooling. It is known that Earth has been totally frozen over at a number of times in the past, a condition called “Snowball Earth” by paleoclimatologists. The big question is how can Earth cool without plunging into another Snowball Earth period? That is the subject of a new paper, “Hydrologic Regulation of Chemical Weathering and the Geologic Carbon Cycle,” published in the journal Science.
Katherine Maher and C. Page Chamberlain, both of Stanford University, report that Earth’s temperature is regulated by a negative feedback between atmospheric CO2 levels and chemical weathering of silicate rocks. This feedback operates over million-year time scales and is strongest when global topography is elevated, as it is today. When mountain rangers are worn and low the mechanism is at its weakest. Here is how they frame the situation in the paper.
Despite substantial changes in solar luminosity, plate tectonics and atmospheric composition, over billions of years temperatures on Earth have remained favorable for liquid water, and by extension, life. A requirement for maintaining such clement conditions is a chemical weathering process that converts atmospheric CO2 and silicate rocks to alkalinity and divalent cations, which are then buried on the seafloor as carbonate minerals. Chemical weathering rates cannot be out of balance with the supply of CO2 from volcanic and metamorphic sources for very long without catastrophic consequences. Fortunately, such imbalances have been infrequent. Yet, Earth’s climate has varied between warm, ice-free conditions and cold, extensively glaciated states, suggesting a climate system with variable regulation. The stability of Earth’s climate thus requires both a negative feedback between chemical weathering rates and temperature, and a mechanism that allows the strength of the feedback, or extent of regulation, to vary. The strength of the feedback is dictated by the functional relationship between the weathering rate and climate, and when balanced against CO2 degassing rates, determines planetary temperatures. Several processes could allow the strength of the feedback to vary, suggesting the mechanisms underlying one of the most profound features in the sculpting of Earth’s history remain unresolved.
The geologic portion of the carbon cycle operates on a much longer time-scale than the cycling of carbon through plants and other living things. Volcanoes supply carbon dioxide that can warm climate. But CO2 also dissolves in water, turning into carbonic acid (H2CO3)that can dissolve rock. The products of this weathering become runoff, flowing down rivers to the sea, where microscopic animals combine them with more carbon dioxide from the air to build hard skeletons. When the animals die all that carbon falls to the bottom of the ocean where it becomes sea floor sediment, which is eventually buried and becomes geologically sequestered for millions of years.
Since spare Earths that can be used for experimentation are in short supply, the authors created a model to test out their hypotheses. The proposed model combines two equations: “(1) a solute transport equation that quantifies weathering-derived solute as a function of the mean fluid travel time in a catchment; and (2) an equation that relates the supply of fresh rock from erosion to the downward propagation of a weathering front.” Fundamental to their calculations is a measure called the Damköhler number.
The Damköhler number is a dimensionless number that compares the mean fluid travel time (Tf ≈ Lϕ/q[yr]) to the time required to reach equilibrium (Teq ≈ Ceq/Rn[yr]), where q[m/yr] is runoff, the reactive flowpath length (Lϕ) is flowpath length (L[m]) times effective porosity (ϕ), Ceq[μmol/L] is the “thermodynamic limit” (i.e., maximum concentration), and Rn[μmol/L/yr] is the reaction rate. The authors then introduce the Damköhler coefficient (Dw), which is modified to account for the supply of fresh minerals through erosion. For the actual equations and other assumptions please refer to the paper.
The bottom line here is to relate temperature to the weathering of rock, which impacts CO2 absorption, as shown in the figure below. Note that a craton is an old and stable part of the continental lithosphere.
The upshot of the curves above is that rocky areas absorb more CO2 and the rate of absorption rises with temperature. Of course it is not really that simple, there are multiple interrelated factors at work here. “We propose that removal of CO2 is regulated by the intensity of the hydrologic cycle, rather than directly by temperature,” the authors state. “Increases in global mean temperatures (GMT) are associated with more precipitation and attendant changes in runoff, although climate model predictions vary widely.”
This uncertainty is to be expected whenever models come into play—models only give a general idea of how the modeled phenomena behave, not exactly how they will respond to specific inputs. What can be concluded from the model is that, during times of global warming and amplification of the hydrologic cycle, weathering fluxes will increase disproportionately between tectonically active and inactive areas. In short, as it gets hotter more carbon dioxide is removed through weathering, and when it gets colder CO2 removal slows down.
We are currently lucky that our planet has a number of relatively young, tall mountain ranges: the Himalaya, the Rockies, and the Andes to name the most prominent ones. New ocean crust is constantly being created at mid-ocean ridges, which means that old crust must either be destroyed or otherwise reduced at the same rate. One of they ways to deal with excess crustal rock is to make mountains. When two continents carried on converging plates collide they crumple and fold under the enormous pressure, creating great mountain ranges.
Currently geologists tell us that all three of the world's newer mountain ranges are still growing, still getting higher. For example, the Sierra Nevada mountain range in North America is growing at a rapid pace—one to two millimeters per year along the entire range, according to researchers. Both the Himalaya and the Andes are also still growing.
As a result, global cooling slows weathering all across the continents and reins in cooling due to mountain building. This connection between weathering and climate is not new, it was first proposed in the early 1990s. Maureen Raymo, of the Lamont-Doherty Earth Observatory, and colleagues proposed that the rise of the Himalayas and the Tibetan Plateau 40 million years ago increased erosion and thus acid weathering. This reduced atmospheric carbon dioxide and cooled the climate to the point that permanent polar icecaps formed at both poles for the first time in 600 million years. Earth remains in an Ice Age today, called the Pleistocene.
But the mountain-building-causes-cooling theory has a number of critics, who asked how the process could have stabilized before driving climate into a deep, permanent icehouse—and nobody had a good answer. The work by Maher and Chamberlain answers those earlier criticisms that the Himalayan scenario would have threatened the planet with runaway cooling. We should all be thankful that Earth's climate system includes such a mechanism or we would be extinct, along with all other higher life on the planet. Conversely, could enough human generated CO2 cause a shift to a much hotter stable temperature range? It is doubtful.
According to Information is Beautiful, we have added 1020 gigatons (Gt) of CO2 since 1850 with another 3000 Gt or so left in our fossil fuel reserves. According to some scientists volcanoes put out about 600 Mt a year, but this is a more or less constant outgassing rate. On geologic time scales, man's contribution is an instantaneous pulse and not of great significance. Indeed, there have been events in the past where nature has released huge volumes of carbon dioxide and other greenhouse gasses.
In “CO2-forced climate thresholds during the Phanerozoic,” Dana L. Royer reviewed the variation of carbon dioxide during the last 550 million years or so. One of the real problems with proxy records is that, as you go further back in time, the resolution of the timescale becomes less precise. Go back several hundred million years and resolving changes on a scale more exact than 10,000 years is problematic. Still, there are some interesting things in that study, particularly having to do with the glaciations preceding the Permian-Triassic extinction, the worst extinction in the history of life on Earth. Is seems that there were two distinct glacial periods prior to the Great Dying.
The older phase begins 326.4 mya and ends 311.7 mya. Evidence for ice at this time comes from western South America and eastern Australia. The second cold phase commences around 302 mya and ends 290 mya. Evidence for ice during this interval is geographically more extensive, coming from South America, Africa, Antarctica, Australia, and India. There is no convincing evidence for ice between these two phases (311.7–302 Ma). But here is what I find interesting:
Between the two glacial phases, CO2 coverage is sparse, but a period of high CO2 (1500 ppm) falls at the end of this interglacial phase. Concomitant with the initiation of the second glacial phase, CO2 quickly drops to below 500 ppm and remains at these levels for the duration of the cold phase. After the termination of the second icy phase (290 Ma), the Earth shifted to a cool state until 267 Ma. During this cool phase, CO2 remained below 500 ppm except for at least one excursion to levels between 500 and 1000 ppm. Directly after the termination of this cool phase, CO2 increased to 1000+ ppm and remained high until the early Triassic.
In other words, a doubling of atmospheric carbon dioxide levels did not terminate the ongoing glacial period. For an excursion to 1000 ppm to register in the proxy record it must have been much longer than anything humanity will generate. This tells us that all of the hand-wringing about drastic and irreversible changes to the climate from human emissions are overblown. By the way, current thinking is that the Great Dying was caused by extensive and long term vulcanism spewing massive amounts of CO2 into the environment.
More over, on shorter time scales CO2 levels are always bouncing around, contrary to what some would have you believe. Friederike Wagner, Bent Aaby, and Henk Visscher, in “Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event,” found a great deal of natural variation. Quoting from their PNAS paper: “In effect, there seems to be every indication that the occurrence of Holocene CO2 fluctuations is more consistent with current observations and models of past global temperature changes than the common notion of a relatively stable CO2 regime until the onset of the Industrial Revolution.”
This could happen again, but it is unlikely.
So it would appear that Earth's current configuration has created some feedbacks that prevent the bottom from falling out of global temperatures when the climate dips into an Ice Age. It is logical to assume that there are similar feedbacks that prevent a runaway in the opposite direction, preventing the global warming catastrophe that the climate change alarmists are always babbling on about. Currently, global temperatures aren't rising and there is conclusive evidence that elevated CO2 levels are good for plants. It seems that all of the bad things predicted by the climate alarmists are not coming true, but the good things they deny are, so let's stop all the scaremongering and deal with the real problems facing humanity.
Be safe, enjoy the interglacial and stay skeptical.