Greenhouse Gas Emissions: The Marginal Contribution Matters

[WeatherRusty]

Didn't say (or didn't mean to say) we'd be all the way there.....as you say, that's dire, as in not at all compatible with life dire.

But we could be as irrevocably committed to it as we would be if the Sun were going nova.....

Once you get past a certain point, all the feedbacks favor further warming, and the Sun has been getting brighter, not dimmer in the 50 million years since the PETM.

The climate system on Earth will not run away with itself. If that were remotely likely it would have happened during the 4.567 billion year history of the planet. It happened to Venus when it lost it's magnetic field due to it's slow rotation. This allowed solar radiation to dissociate water molecules in the upper atmosphere of the planet with the hydrogen escaping to space. Eventually Venus lost all it's water and any oceans it may have had early on.

On Earth the oceans absorb CO2 and deposit it as sediment on the sea floor, over the eons burying the carbon as sedimentary rock. On Venus, billions of years of CO2 belched into the atmosphere from volcanic and other processes remains in the atmosphere creating a massive greenhouse effect by both longwave radiation absorption and very high atmospheric pressure.

The Earth's magnetic field defects the solar wind and permits the sustainability of oceans. We know CO2 has exceeded 1000ppm in the deep past on Earth. It can get hot for life on Earth, but nothing like the 900F at the Venusian surface.

SEE

[dabize]

If that were remotely likely it would have happened during the 4.567 billion year history of the planet.

It happened to Venus when it lost it's magnetic field due to it's slow rotation. This allowed solar radiation to dissociate water molecules in the upper atmosphere of the planet with the hydrogen escaping to space. Eventually Venus lost all it's water and any oceans it may have had early on.

The Earth's magnetic field defects the solar wind and permits the sustainability of oceans. We know CO2 has exceeded 1000ppm in the deep past on Earth. It can get hot for life on Earth, but nothing like the 900F at the Venusian surface.

SEE

Interesting - I can see how the lack of a magnetic field would preclude life, and development of an oxygen atmosphere. The loss of water mechanism via cosmic ray exposure and the rest of the downstream events from that do seem to explain Venus.

But this does not support your first line, which fails to take into account the 3-5% increase in solar luminosity since Paleozoic/Mesozoic events. Also, the fact that Earth has plenty of H2O and thus a ready source of water vapor (an effective greenhouse gas), might make us wish we'd lost some of that water........although I suppose we'd be dead either way.

To me, a runaway event featuring increased water vapor, the release of biomass sources of CO2 and CH4 and the stripping of CO2 from surface minerals via heating would be sufficiently bad to not care about the details of whether the mechanism is identical to Venus.

[WeatherRusty]

Interesting - I can see how the lack of a magnetic field would preclude life, and development of an oxygen atmosphere. The loss of water mechanism via cosmic ray exposure and the rest of the downstream events from that do seem to explain Venus.

But this does not support your first line, which fails to take into account the 3-5% increase in solar luminosity since Paleozoic/Mesozoic events. Also, the fact that Earth has plenty of H2O and thus a ready source of water vapor (an effective greenhouse gas), might make us wish we'd lost some of that water........although I suppose we'd be dead either way.

To me, a runaway event featuring increased water vapor, the release of biomass sources of CO2 and CH4 and the stripping of CO2 from surface minerals via heating would be sufficiently bad to not care about the details of whether the mechanism is identical to Venus.

Solar evolution guarantees the Earth will experience surface temperatures hot enough to boil away the oceans. That will produce a run away global warming, but not for at least another 1 billion years. It is estimated that multi-cellular life has reached about the halfway point for viability on Earth.

Also, remember...water vapor mixing ratio follows ambient temperature. Water vapor acts as a powerful greenhouse gas to enhance any warming in an act of positive feedback. This feedback is partially mitigated by the negative lapse rate feedback concurrently induced (reducing greenhouse effect..net positive) and also by changes in cloudiness. Water vapor entering the atmosphere will not by itself cause a run away condition so long as an equal amount continues to precipitates out as what evaporates in.

In the deep past, climates warmer than 10C above current conditions have occurred. The climate system did not run away. Over the next several centuries humans stand to recreate the conditions of the PETM of 55 million years ago. The climate did not run away then either. Regardless of the global warming event we are currently immersed in (and causing), eventually the Earth will return to the climate conditions forced by the dynamic equilibrium between solar radiation and the Earth's natural production of greenhouses gases.

Now to the bold text. As it just so happens, atmospheric CO2 content has fallen in step with the warming sun over the past several hundred million years. Good thing too, imagine how warm the Earth would be today if CO2 measured in parts per million were near 800ppm as they were during the period you refer to.

We are really just incredibly lucky to be here in the first place. The Earth just so happens to reside in a narrow region just the right distance from the Sun to sustain temps suitable for life and for water to exist near it's triple point (gas, liquid, solid). We have a moon which stabilizes the Earth's tendency to wobble wildly about it's rotational axis. This is a major reason we have stable seasons on Earth. A few chance big asteroid impacts have randomly shaped the evolution of life on Earth. If any of of these and many more coincidences had not occurred we would most likely not be here. NO, we wouldn't be here.

One of those chance happenings is that over the past several hundred million years CO2 levels have declined. Tectonic process involving mountain building and the positions of the continents, the amount of water runoff from the continents to the seas. The nature of life itself. The temperature, cooler conditions slowing chemical rock weathering in concert with all the above determine the amount of CO2 in the atmosphere. We are just lucky things have taken place just the exact way they have, including a period of low atmospheric CO2 coinciding with a warming Sun. If that had not been the case, the heat of the Cretaceous Period would have built even higher with the warming of the Sun, and who knows maybe the dinosaurs would have survived the asteroid to remain the predominant form of life on the planet.

[dabize]

Solar evolution guarantees the Earth will experience surface temperatures hot enough to boil away the oceans. That will produce a run away global warming, but not for at least another 1 billion years. It is estimated that multi-cellular life has reached about the halfway point for viability on Earth.

Also, remember...water vapor mixing ratio follows ambient temperature. Water vapor acts as a powerful greenhouse gas to enhance any warming in an act of positive feedback. This feedback is partially mitigated by the negative lapse rate feedback concurrently induced (reducing greenhouse effect..net positive) and also by changes in cloudiness. Water vapor entering the atmosphere will not by itself cause a run away condition so long as an equal amount continues to precipitates out as what evaporates in.

In the deep past, climates warmer than 10C above current conditions have occurred. The climate system did not run away. Over the next several centuries humans stand to recreate the conditions of the PETM of 55 million years ago. The climate did not run away then either. Regardless of the global warming event we are currently immersed in (and causing), eventually the Earth will return to the climate conditions forced by the dynamic equilibrium between solar radiation and the Earth's natural production of greenhouses gases.

Now to the bold text. As it just so happens, atmospheric CO2 content has fallen in step with the warming sun over the past several hundred million years. Good thing too, imagine how warm the Earth would be today if CO2 measured in parts per million were near 800ppm as they were during the period you refer to.

We are really just incredibly lucky to be here in the first place. The Earth just so happens to reside in a narrow region just the right distance from the Sun to sustain temps suitable for life and for water to exist near it's triple point (gas, liquid, solid). We have a moon which stabilizes the Earth's tendency to wobble wildly about it's rotational axis. This is a major reason we have stable seasons on Earth. A few chance big asteroid impacts have randomly shaped the evolution of life on Earth. If any of of these and many more coincidences had not occurred we would most likely not be here. NO, we wouldn't be here.

One of those chance happenings is that over the past several hundred million years CO2 levels have declined. Tectonic process involving mountain building and the positions of the continents, the amount of water runoff from the continents to the seas. The nature of life itself. The temperature, cooler conditions slowing chemical rock weathering in concert with all the above determine the amount of CO2 in the atmosphere. We are just lucky things have taken place just the exact way they have, including a period of low atmospheric CO2 coinciding with a warming Sun. If that had not been the case, the heat of the Cretaceous Period would have built even higher with the warming of the Sun, and who knows maybe the dinosaurs would have survived the asteroid to remain the predominant form of life on the planet.

Really nice response, Rusty. Your ability to connect the details with their big picture consequences in a coherent readable post is a truly valuable resource here.

I suppose I am most concerned that we are threading the needle on this, hoping that the uncertainties involved in painting this scenario all fall in our favor.

Even without Venus (and I remain convinced that this type of runaway could happen, if the uncertainties do not go our way - see Hansen's "Storms of our Grandchildren"), there are plenty of factors that could take us out of the "biosphere" range with a BAU approach to AGW.

This is best exemplified by the recent SkS post that I linked (in a new thread) describing the likelihood that even PETM-like conditions would not be survivable.

[cmc0605]

WeatherRusty is pretty much spot on with his description of Earth's current inability to generate a runaway greenhouse.

In fact, the physics that determine when a planet "runs away" (whether it be Earth, Venus, or some new exoplanet) is almost completely independent of the CO2 concentration in the atmosphere, and depends primarily on the amount of sunlight the body absorbs.

Normally, any planet with a liquid ocean will generate a water vapor feedback, with the concentration of vapor growing with temperature (roughly to scale with the Clausius-Clapeyron equation, though the dynamics helps govern the degree of subsaturation). The growth of atmsopheric opacity with temperature wipes away a lot of the dependence between the outgoing longwave radiation (OLR) and surface temperature, forcing it to me much more linear than T^⁴. A high enough temperatures, the OLR becomes decoupled from the surface temperature altogether and eventually an observer from space would not see any increase in OLR even if the surface-atmosphere column increased in T.

But to sustain a true runaway, you need an amount of sunlight absorbed that is greater than this threshold OLR. If the sunlight is less than this limiting OLR, the system can reach an equilibrium even for a high climate sensitivity. But if the sunlight exceeds this limiting OLR, than equilibrium is impossible until the water vapor feedback ceases (when the oceans are boiled away) or when the planet becomes so hot that it radiates energy to space in the visible spectrum where atmsopheric opacity is weak.

The eventual increase in CO2 on Venus is not so much a cause of this scenario, as much as a result, due to the weathering feedback described above by Rusty (although the description is a bit confused since the timescales between the silicate weathering feedback of carbon is much longer than the ocean feedbacks of interest, to say, glacial-interglacial cycles). The next step in the process after liquid water is incomaptible at the surface is that the upper atmosphere is now very moist and hydrogen/oxygen can become photodissociated by UV radiation. The hydrogen is lost to space and the process becomes irreversible, and without liquid water there is no major chemical reactions that can remove CO2 from the atmosphere (so any input from volcanoes for example just stays there).

[WeatherRusty]

WeatherRusty is pretty much spot on with his description of Earth's current inability to generate a runaway greenhouse.

In fact, the physics that determine when a planet "runs away" (whether it be Earth, Venus, or some new exoplanet) is almost completely independent of the CO2 concentration in the atmosphere, and depends primarily on the amount of sunlight the body absorbs.

Normally, any planet with a liquid ocean will generate a water vapor feedback, with the concentration of vapor growing with temperature (roughly to scale with the Clausius-Clapeyron equation, though the dynamics helps govern the degree of subsaturation). The growth of atmsopheric opacity with temperature wipes away a lot of the dependence between the outgoing longwave radiation (OLR) and surface temperature, forcing it to me much more linear than T^⁴. A high enough temperatures, the OLR becomes decoupled from the surface temperature altogether and eventually an observer from space would not see any increase in OLR even if the surface-atmosphere column increased in T.

But to sustain a true runaway, you need an amount of sunlight absorbed that is greater than this threshold OLR. If the sunlight is less than this limiting OLR, the system can reach an equilibrium even for a high climate sensitivity. But if the sunlight exceeds this limiting OLR, than equilibrium is impossible until the water vapor feedback ceases (when the oceans are boiled away) or when the planet becomes so hot that it radiates energy to space in the visible spectrum where atmsopheric opacity is weak.

The eventual increase in CO2 on Venus is not so much a cause of this scenario, as much as a result, due to the weathering feedback described above by Rusty (although the description is a bit confused since the timescales between the silicate weathering feedback of carbon is much longer than the ocean feedbacks of interest, to say, glacial-interglacial cycles). The next step in the process after liquid water is incomaptible at the surface is that the upper atmosphere is now very moist and hydrogen/oxygen can become photodissociated by UV radiation. The hydrogen is lost to space and the process becomes irreversible, and without liquid water there is no major chemical reactions that can remove CO2 from the atmosphere (so any input from volcanoes for example just stays there).

I might also add, on Venus the loss of oceans dealt a death knell to any plate tectonics the planet may have had. Subducted sea water on Earth lubricates the plate boundaries allowing for easier movement of crustal features and continents. Most of the dynamic processes going on at Venus and Mars have long since died out, but I suppose these planet could still produce hot spot volcanism. Even then, both planets being somewhat smaller in size and mass than Earth will have cooler cores. Just another of the amazing set of coincidences which allows for a vibrant and dynamic, life filled Earth.