Our Goldilocks Climate

haze_archean_2_cropped_2In the fairy tale, Goldilocks entered the three bears’ house to find one bowl of soup too hot, another too cold, and one just right for her to eat. A new study of our planetary history suggests that since its beginning our climate has been self-regulating to avoid extremes, with much less variability in temperature and oceanic pH than previously thought.

An overview of the finding comes from an article in Phys.org and is followed by excerpts from the paper itself published in PNAS.

Introductory Comments from Phys.org article Earth’s stable temperature past suggests other planets could also sustain life  April 2, 2018, University of Washington. Excerpts with my bolds.

Theories about the early days of our planet’s history vary wildly. Some studies have painted the picture of a snowball Earth, when much of its surface was frozen. Other theories have included periods that would be inhospitably hot for most current lifeforms to survive.

New research from the University of Washington suggests a milder youth for our planet. An analysis of temperature through early Earth’s history, published the week of April 2 in the Proceedings of the National Academy of Sciences, supports more moderate average temperatures throughout the billions of years when life slowly emerged on Earth.

“Our results show that Earth has had a moderate temperature through virtually all of its history, and that is attributable to weathering feedbacks—they do a good job at maintaining a habitable climate,” said first author Joshua Krissansen-Totton, a UW doctoral student in Earth and space sciences.

To create their estimate, the researchers took the most recent understanding for how rocks, oceans, and air temperature interact, and put that into a computer simulation of Earth’s temperature over the past 4 billion years. Their calculations included the most recent information for how seafloor weathering occurs on geologic timescales, and under different conditions.

Seafloor weathering was more important for regulating temperature of the early Earth because there was less continental landmass at that time, the Earth’s interior was even hotter, and the seafloor crust was spreading faster, so that was providing more crust to be weathered,” Krissansen-Totton said.

The paper is by Joshua Krissansen-Totton el al., Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model PNAS (2018). Excerpts with my bolds.

The existence of a negative feedback to balance the carbon cycle on million-year timescales is undisputed. Without it, atmospheric CO2 would be depleted, leading to a runaway icehouse, or would accumulate to excessive levels (34). However, the relative importance of continental and seafloor weathering in providing this negative feedback, and the overall effectiveness of these climate-stabilizing and pH-buffering feedbacks on the early Earth are unknown.

In this study, we apply a geological carbon cycle model with ocean chemistry to the entirety of Earth history. The inclusion of ocean carbon chemistry enables us to model the evolution of ocean pH and realistically capture the pH-dependent and temperature-dependent kinetics of seafloor weathering. This is a significant improvement on previous geological carbon cycle models (e.g., refs. 12 and 35) that omit ocean chemistry and instead adopt an arbitrary power-law dependence on pCO2 for seafloor weathering which, as we show, overestimates CO2 drawdown on the early Earth. By coupling seafloor weathering to Earth’s climate and the geological carbon cycle, we calculate self-consistent histories of Earth’s climate and pH evolution, and evaluate the relative importance of continental and seafloor weathering through time. The pH evolution we calculate is therefore more robust than that of Halevy and Bachan (29) because, unlike their model, we do not prescribe pCO2 and temperature histories.

The climate and ocean pH of the early Earth are important for understanding the origin and early evolution of life. However, estimates of early climate range from below freezing to over 70 °C, and ocean pH estimates span from strongly acidic to alkaline. To better constrain environmental conditions, we applied a self-consistent geological carbon cycle model to the last 4 billion years. The model predicts a temperate (0–50 °C) climate and circumneutral ocean pH throughout the Precambrian due to stabilizing feedbacks from continental and seafloor weathering. These environmental conditions under which life emerged and diversified were akin to the modern Earth. Similar stabilizing feedbacks on climate and ocean pH may operate on earthlike exoplanets, implying life elsewhere could emerge in comparable environments.

Schematic of carbon cycle model used in this study. Carbon fluxes (Tmol C y−1) are denoted by solid green arrows, and alkalinity fluxes (Tmol eq y−1) are denoted by red dashed arrows. The fluxes into/out of the atmosphere–ocean system are outgassing, Fout, silicate weathering, Fsil, carbonate weathering, Fcarb, and marine carbonate precipitation, Pocean. The fluxes into/out of the pore space are basalt dissolution, Fdiss, and pore-space carbonate precipitation, Ppore. Alkalinity fluxes are multiplied by 2 because the uptake or release of one mole of carbon as carbonate is balanced by a cation with a 2+ charge (typically Ca2+). A constant mixing flux, J (kg y−1), exchanges carbon and alkalinity between the atmosphere–ocean system and pore space.

The dissolution of basalt in the seafloor is dependent on the spreading rate, pore-space pH, and pore-space temperature (SI Appendix A). This formulation is based on the validated parameterization in ref. 36. Pore-space temperatures are a function of climate and geothermal heat flow. Empirical data and fully coupled global climate models reveal a linear relationship between deep ocean temperature and surface climate (36). Equations relating pore-space temperature, deep ocean temperature, and sediment thickness are provided in SI Appendix A.

Carbon leaves the atmosphere–ocean system through carbonate precipitation in the ocean and pore space of the oceanic crust. At each time step, the carbon abundances and alkalinities are used to calculate the carbon speciation, atmospheric pCO2, and saturation state assuming chemical equilibrium. Saturation states are then used to calculate carbonate precipitation fluxes (SI Appendix A). We allow calcium (Ca) abundance to evolve with alkalinity, effectively assuming no processes are affecting Ca abundances other than carbonate and silicate weathering, seafloor dissolution, and carbonate precipitation. The consequences of this simplification are explored in the sensitivity analysis in SI Appendix C. We do not track organic carbon burial because organic burial only constitutes 10–30% of total carbon burial for the vast majority of Earth history (40), and so the inorganic carbon cycle is the primary control.

We conclude that current best knowledge of Earth’s geologic carbon cycle precludes a hot Archean. Our results are insensitive to assumptions about ocean chemistry, internal evolution, and weathering parameterizations, so a hot early Earth would require some fundamental error in current understanding of the carbon cycle. Increasing the biotic enhancement of weathering by several orders of magnitude as proposed by Schwartzman (60) does not produce a hot Archean because this is mathematically equivalent to zeroing out the continental weathering flux (Fig. 4). In this case the temperature-dependent seafloor weathering feedback buffers the climate of the Earth to moderate temperatures (SI Appendix, Fig. S14). Dramatic temperature increases (or decreases) due to albedo changes also do not change our conclusions due to the buffering effect of the carbon cycle (see above). If both continental and seafloor weathering become supply limited (e.g., refs. 49 and 61), then temperatures could easily exceed 50 °C. However, in this case the carbon cycle would be out of balance, leading to excessive pCO2 accumulation within a few hundred million years unless buffered by some other, unknown feedback.

The only way to produce Archean climates below 0 °C in our model is to assume the Archean outgassing flux was 1–5× lower than the modern flux (SI Appendix, Fig. S12). However, dramatically lowered Archean outgassing fluxes contradict known outgassing proxies and probably require both a stagnant lid tectonic regime and a mantle more reduced than zircon data suggest, which lowers the portion of outgassed CO2 (SI Appendix C). Moreover, even when outgassing is low, frozen climates are not guaranteed (SI Appendix, Fig. S12).

We observe that modeled temperatures are relatively constant throughout Earth history, with Archean temperatures ranging from 271 to 314 K. The combination of continental and seafloor weathering efficiently buffers climate against changes in luminosity, outgassing, and biological evolution. This temperature history is broadly consistent with glacial constraints and recent isotope proxies (Fig. 3D). The continental weathering buffer dominates over the seafloor weathering buffer for most of Earth history, but in the Archean the two carbon sinks are comparable (SI Appendix, Fig. S1). Indeed, if seafloor weathering were artificially held constant, then continental weathering alone may be unable to efficiently buffer the climate of the early Earth—the temperature distribution at 4.0 Ga extends to 370 K, and the atmospheric pCO2 distribution extends to 7 bar (SI Appendix, Fig. S3).

In our nominal model, the median Archean surface temperature is slightly higher than modern surface temperatures. If solar evolution were the only driver of the carbon cycle, then Archean temperatures would necessarily be cooler than modern temperatures; weathering feedbacks can mitigate this cooling but not produce warming. Warmer Archean climates are possible because elevated internal heat flow, lower continental land fraction, and lessened biological enhancement of weathering all act to warm to Precambrian climate. These three factors produce a comparable warming effect (SI Appendix, Fig. S17A and Appendix C), although the magnitude of each is highly uncertain and so temperate Archean temperatures cannot be uniquely attributed to any one variable.

Conclusions

The early Earth was probably temperate. Continental and seafloor weathering buffer Archean surface temperatures to 0–50 °C. This result holds for a broad range of assumptions about the evolution of internal heat flow, crustal production, spreading rates, and the biotic enhancement of continental weathering. Even in extreme scenarios with negligible subaerial Archean land and high methane abundances, a hot Archean (>50 °C) is unlikely. Sub-0 °C climates are also unlikely unless the Archean outgassing flux was unrealistically lower than the modern flux.

The seafloor weathering feedback is important, but less dominant than previously assumed. Consequently, the early Earth would not have been in a snowball state due to pCO2 drawdown from seafloor weathering. In principle, little to no methane is required to maintain a habitable surface climate, although methane should be expected in the anoxic Archean atmosphere once methanogenesis evolved (ref. 62, chap. 11).

Ignoring transient excursions, the pH of Earth’s ocean has evolved monotonically from 6.6+0.6−0.4 at 4.0 Ga (2σ) to 7.0+0.7−0.5 at 2.5 Ga (2σ), and 8.2 in the modern ocean. This evolution is robust to assumptions about ocean chemistry, internal heat flow, and other carbon cycle parameterizations. Consequently, similar feedbacks may control ocean pH and climate on other Earthlike planets with basaltic seafloors and silicate continents, suggesting that life elsewhere could emerge in comparable environments to those on our early planet.

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