What has prevented runaway greenhouse effect in the past.?

Increasing CO2 concentration cannot cause a “runaway greenhouse effect “.

CO2 contributes to the greenhouse effect by absorbing infra-red heat reflected back up from the earth towards space. As you increase the CO2 concentration then you increase the absorption... but this relationship is not linear, it is logarithmic. For each doubling of CO2 concentration you get the same amount of absorption as the previous doubling. i.e if increasing co2 concentration from 100ppm to 200ppm gives X watts of absorption, then increasing from 200ppm to 400ppm will absorb a further X watts. Increasing from 400ppm to 800 ppm absorbs another X watts and so on.

Increasing CO2 concentration in this way quickly reaches a maximum absorption rate at which point adding more CO2 has very little effect. It’s ‘greenhouse’ effect can’t runaway, there’s a ceiling.

This is very good reading on how CO2 works in the atmosphere. http://brneurosci.org/co2.html

We know that the earth has warmed and cooled many times in the past. We know the Milankovitch cycle is very important in this respect. We’re now also beginning to learn about solar wind intensity changing the shape of our atmosphere and thereby influencing climate, and we know that changing cosmic ray intensity influences the rate of cloud formation.

These are huge external forces working on the earth and are far more likely to influence our climate than a piffling bit of CO2. The worlds temperatures will rise and fall in the future. CO2 will have nearly nothing to do with it.

In general, the climate has a driver: this can be changes in orbit (Milankovitch cycles), changes in the solar forcing (sun getting hotter over time, or Maunder minimum), or human CO2 emissions.

The climate also has feedbacks: the big one is water vapor: as the climate warms, more water evaporates, leading to more water vapor, leading to more warming. The CO2 feedback occurs because warmer oceans can hold less CO2, and this feedback is thought to have contributed to the warming started by the Milankovitch driver in glacial/interglacial cycles. Ditto ice sheet retreat. (note all these feedbacks operate both ways: as the climate cools, water condenses, oceans hold more CO2, ice sheets grow).

Of course, as the earth warms, it radiates more heat (Stefan-Boltzmann law). In order to have runaway warming, the feedbacks need to outweigh the Stefan-Boltzmann effect.

Making this more complicated is that systems have multiple equilibrium points, where certain feedbacks can occur only over limited ranges. For example, the ice sheet retreat feedback can only keep going until there are no more ice sheets. Even the most famous case of runaway warming, Venus, eventually reached a point where it reached a new equilibrium (albeit, a really really hot and nasty one).

Generally, climate scientists today do not worry about "runaway warming". They think the positive feedbacks are there, but, like a geometric series (1 + 1/2 + 1/4 + 1/8 + ...) a given impulse of a driver will lead to an ever diminishing cycle of feedbacks, with a finite total temperature change. This is what the term "climate sensitivity" is used to define: a doubling of CO2 leads to about 1 degree C direct warming, but because of feedbacks the systems new temperature will be larger. Best estimates of climate sensitivity range from 2 to 4.5 or so.

Considering the scientific fact that burning fossil fuels releases CO2 , and the fact that humans have been burning the crud for more than 100 years,now the negative effects are being experienced. We have "broken" the system. Now the danger comes from erratic weather patterns. As the heat energy in the atmosphere increases,so does it's effect on weather. Heat,when clashing with colder air,create storms. More heat, stronger storms. More heat,more evaporation from the oceans,means more rainfall amounts. If you are on a swing, how hard of a push would it take before swinging would become unstable and dangerous? We are 'pushing' the climate beyond it's 'normal' parameters. The problem is, the climate can change very suddenly,even if it seems to stable in your short lifetime. Why risk it?

In the past, the carbon cycle has been in balance. Orbital and solar cycles are 2 major forces controlling how warm the earth gets. As it gets warmer because of these cycles, decay also increases adding add CO2 to the atmosphere. The warmer it is and the more CO2, the more plants grow, absorbing and storing the CO2 until they die. Most of the carbon is then released back into the air, but some is stored in the earth. As the orbital and solar cycles cause cooling, the decay process slows. Not as much CO2 is released and the earth cools more. More plants die and the ice age set in. This process takes thousands of years typically. This is why atmospheric CO2 typically lags behind warming. This is well known and CO2 concentrations correlate very well with mean global temperature (as best as scientists can tell using proxy data).

Since the industrial revolution, humans have been removing CO2 stored in the earth as fossil fuels and burning it, releasing CO2 into the air. This has never been the case in the history of the earth. Now humans are releasing ~30,000,000,000 tons of CO2 into the air annually and the concentrations are building up, not only from decay, but from human activity. CO2 is highly correlated to global mean temperature. Temperature used to cause an increase the amount of CO2 in the air, but because of human activity, the increase of CO2 in the atmosphere has cause temperature to increase (about 0.7 C globally on average).

The earth will adjust to a new equilibrium - there is no doubt of that and the biggest factors are still the orbital and solar cycles that control the ice ages. However, human life spans are very short relative to these cycles. The thought is to reduce our impact on the warming as much as possible to get back into the "natural" balance.

The balance is important because our ecosystems are better able to adapt to the natural rate of change than the accelerated rate of change. These systems provide us with food, shelter, clothing and water. The accelerated global warming caused by humans will result in a change in the earth's climates, and have some potentially detrimental effects on the systems we need to support our life in a comfortable way on the planet. We are just "tweaking" the temperature at the edges, but those tweaks can have enormous impacts in the short term (a few human lifetimes) that can reduce the ability of our species to thrive on the planet.

I believe nobody can say for sure what the climate will do in the future. If we knew that, we could prepare for whatever's next. The fact that the earth warms and cools explains why dinosaur bones have been found in Canada's high Arctic. They once lived there, but at that time there weren't any man-made carbon emmissions. I guess at times there has actually been runaway warming in the past, but I don't think it was the greenhouse effect.

Clearly, CO2 has not been the driver for climates. The Milankovitch Cycle theory fits the evidence much better. I am sometimes amazed to see that there are climatologists that discount it or assume CO2 must be driving the current climate while they discount previous climate trends. It's like I woke up in the twilight zone. It is kind of like ants assuming car exhaust is driving the cars that rumble by their world. The little ant scientist probably theorize that the car exhaust has CO2 and is warm and since cars emit it, what else could drive those cars. OK I am exaggerating a little, very little.

The green house gas is not running away. In dealing with hazardous gas U look at the oxygen level and not the CO2. The heavier gas will displace oxygen. There has been no loss in oxygen. It is 20.9% of our atmosphere and has not changed. The environmentalist are worried about CO2 at 380 ppm. That is .000380% of our atmosphere.The plants have done a great job with the CO2 .

Methane is a light gas and there is not a trace of it. Most is alarmist that do not know gas.

Good question.

I'd guess the extra warmth promotes vegetation, soaking up C02 and reducing the greenhouse effect. It would seem logical to me.

The Greenhouse effect is a theory not science. If you actually do some research you will find there is VASTLY more data that proves a flawed and very weak theory.

Lock down and handcuffs.