The planet Venus has an atmosphere containing 92% carbon dioxide and an atmospheric pressure nearly 100 times that of Earth. As a consequence, the greenhouse effect on Venus is enormous. Without that greenhouse effect the planet would be several hundred degrees less hot than it is at the surface. The Earth has very little CO2 in comparison, only 396 parts per million of the atmosphere by molecule or 0.04% by volume. How can such a seemingly feable amount of the gas be a concern for us here on Earth? Well for one thing, the total greenhouse effect on Earth amounts to 33C rather than 100s of degrees on Venus, and CO2 accounts for maybe 6C degrees on it own, while water vapor and clouds make up about 26C degrees. Not much compared to Venus, but quantity does not tell the whole story.

The absolute quantity of a greenhouse gas is less important than the marginal difference increase as a percentage of the total. For each doubling of CO2 3.7 watts per square meter (3.7W/m^2) of additional energy is retained within the troposphere (the lowest level of the atmosphere). We will double CO2 over pre-industrial levels ( 280 part per million / 560ppm) by mid 21st century. By that time 3.7W/m^2 of additional warming energy will be radiated to the surface by CO2. This will warm the surface by 1.2C after the surface temperature reaches thermal equilibrium with the new “forcing”. This is basic physics. In actuality there will be more warming than that, how much more we are not certain because of feedbacks within the climate system. One thing we do know, the last time it was that warm or warmer, sea levels were at least 10s of feet higher than today, and that is no joke.

I was recently asked what I thought were the causes of climate change before our present time. Climate has changed many, many times over the course of Earth’s history as a planet. I was also asked about warming on Mars, the next planet out from the Sun beyond Earth. The following was my response.

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What warming on Mars? There certainly will be regional and hemispheric changes on Mars as it revolves about the Sun, just as there are on Earth. Also, the eccentricity of Mar’s orbit is greater than Earth’s and the orbital period is nearly twice as long. One thing is for sure, any recent warming at Earth or Mars is not due to any increase in solar output, or what we term TSI (Total Solar Irradiance) which we measure with high accuracy from satellite.

When asking about prehistoric climate change it is important to reference the time periods involved. Over geologic time scales (10s of millions to billions of years) changes to the positions of the continental masses, topography, ocean circulation patterns, atmospheric composition, planetary albedo and a slow increase in solar radiance as the Sun ages all must be considered. On shorter time scales orbital parameters (eccentricity, obliquity and precession), slowly change in time with overlapping phases ranging from 22 to 100 thousand years. Periods of enhanced or reduced volcanism and slight changes in solar irradiance can cause lesser impact climate change events like the Medieval Warm Period and Little Ice Ages.

When speaking of climate sensitivity it is important to note that what is being referred to specifically relates to a single doubling of CO2 alone. In other words, it estimates the total warming to be expected from a natural CO2 concentration of 280ppmv increased to 560ppmv, once thermal equilibrium is reached with the forcing and all short term feedbacks have played out.

However, this is not meant to say CO2 concentration will stop rising at 560ppm, our mitigation response if any will impact greatly on that. This also does NOT include carbon feedback from the melting of the northern permafrost, the melting of arctic methane clathrates, tropical deforestation or any of the other anthropogenic related greenhouse gases or black carbon etc. All these additional contributions are on top of a single doubling of CO2 to produce a total anthropogenic forcing greater than 3.7Wm^2 to which global temperature should respond proportionately. This additional forcing is what drives the potential for temps at 4C and above.

With an increase of 2-3C the Earth would be in a temperature regime not experience for at least 15 million years. We can expect that much warming due to the single doubling of CO2 given the published estimates of climate sensitivity. How this triggers the aforementioned feedback issues potentially places the world in uncharted water, but we can be quite sure of one thing….the Earth will no longer possess a permanent northern ice cap at those sustained temperatures.

Those who acknowledge AGW, but attribute only minor contribution to the past 150 years of warming to it, are either ignorant or in denial of the science. In terms of radiative forcing, nothing understood to have occurred or likely to occur comes close to the radiative forcing produced by accumulating atmospheric CO2. Maybe some deeply mysterious mechanism, currently unknown or poorly understood factors could come to the rescue, but science is not about what we don’t know, it’s works with what we do know.

The array of scientific specialties which lead us to conclude that past warming has been caused predominantly by accumulating greenhouse gases and land use practices by humans, strongly implies that the future climate will continue to warm. The best scientific assessment places the degree of warming to expect from a doubling of CO2 or it’s equivalent radiative forcing at between 2C and 4.5C. That amount of warming positions the Earth’s climate significantly warmer than anything known to have occurred for at least several millions of years.

Those who argue for a very low climate sensitivity (<2C) need only to look to the arctic to see positive feedback at work. Is some of what is happening in the arctic due to factors other than a general warming of the planet? Of course, natural variability is always at work, but let’s not loose sight of what science says is the driving factor which has set this into motion. That driving influence continues to move in only one direction and will carry the effects of natural variability on it’s back.

From all indications there will be no functionally effective, multinational agreement to radically and precipitously ratchet down carbon emissions any time soon. Anything less than that just won’t do.

So we will be at the mercy of how rapidly the free market and government controlled economies can produce, export and scale up clean, renewables to global usage, Anything less than something like 80% of global emission at the 1990 level will not suffice. The reason for that is CO2 concentration/rate of accumulation is already well past the point where at least 2C of warming will occur relative to pre-industrial times when CO2 hadn’t exceeded 280ppm. It is now over 390ppm and rising at a rate of 2ppm per annum.

It’s been at least 3 million years since CO2 levels exceeded 400ppm and temperature was as warm as we stand to soon experience. Atmospheric CO2 of over 400ppm coincided with global temps 4-6C warmer than today going back 20 million years ago. Since the mid Cretaceous Period about 100 million years ago when CO2 hovered around 600ppm and global temps were as much as 10C warmer than today, CO2 has experienced a general decline in the atmosphere to reside generally between 180ppm and 280ppm over the past 3 million years of northern hemisphere periodic glaciation.

Natural geologic process regulate the amount of CO2 resident in the atmosphere on multi-million year time frames. Once in the atmosphere CO2 level acts to regulate surface temperature on Earth much the same way as a thermostat does. How it finds itself into the atmosphere in the first place is dependent on the ambient temperate and how this affects the chemistry of rock weathering and CO2 concentration within the oceans. Warmer seas hold less CO2 in solution, so the warmer it becomes due to an enhanced rate of rock weathering, the less capable the oceans are of scrubbing that CO2 out of the atmosphere. The added CO2 then acts to enhance the greenhouse effect. Atmospheric CO2 acts therefore as both a feedback to warming and also a cause of warming. This is a classic positive feeback loop.

Now mankind comes along and becomes a source for introducing CO2 to the atmosphere at a much greater rate than what occurs by the process of natural rock weathering. Natural CO2 sinks were in balance with natural sources of CO2 given overall conditions. Humans are now responsible for about 3% of the total CO2 engaged in the natural carbon cycle as the Earth system literally inhales and exhales CO2. That 3% is the marginal difference which has disrupted the natural balance. The Earth, given current geological conditions, is capable of inhaling only so much, which used to be close to equilibrium with how much it was exhaling. That extra 3% contributed by man’s activities is building in the Earth’s oceans and atmosphere and it will remain there for 100′s to 1,000′s of years, only slowly falling to natural background levels as natural process eventually catch up with the imbalance. The higher that CO2 concentration becomes in parts per million, the longer will be required to reestablish balance over coming decades and centuries.

That basically in a nutshell is what has and is going on according to scientific investigation.

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.

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.

Greenland ice melt seen at lower temperatures: study Sun, 11 Mar 2012 15:00:07 -0400
LONDON (Reuters) – The complete melt of the Greenland ice sheet could occur at lower global temperatures than previously thought, a study in the journal Nature Climate Change showed on Sunday, increasing the threat and severity of a rise in sea level.

INDIANA U. (US) — Over the next century, the climate will change more than 100 times faster than the rate at which species can adapt, according to new research.

Snakes struggle to keep pace with climate.

PENN STATE (US) — Engineering our way out of global climate warming may not be as easy as simply reducing the incoming solar energy, a research team concludes.

Designing the approach to control both sea level rise and rates of surface air temperature changes requires a balancing act to accommodate the diverging needs of different locations.

Climate conflict: Sea level vs. surface temp.

As hot weather continues to bake much of the county’s midsection, the National Weather Service issued heat advisories and extreme heat warnings for parts of 15 southern states yesterday. Across states like Texas, Oklahoma, Arkansas, and Mississippi, temperatures set new daily high records, adding to the tally of thousands of temperature records that were broken so far this summer across a wide swatch of the country.

Here are some of the most striking records set during the past week:

On Wednesday, the mercury hit 115°F in Fort Smith, Arkansas, which shattered the all-time record of 107 degrees, set back in 1896. Fayetteville, Ark. hit 110°F on Wednesday, which broke the 47-year-old record of 102°F. Little Rock also set a new all-time record high temperature, at 114°F.

July 2011 was the warmest month on record in Lubbock, Texas. This year, the city has already had at least 34 days with temperatures at or exceeding 100°F, which breaks the previous record of 29 days set during the “Dust Bowl” era. With most of August still to come, this new record will probably end up even higher.

Tyler, Texas hit a record-breaking 36-day streak of triple-digit temperatures on August 2. Dallas-Ft. Worth has had 34 straight days with temperatures above 100°F (the second longest streak for the area).

It was also the hottest July on record in Oklahoma, where the state’s average temperature was 89.1°F. That’s more than seven degrees above average for July, and 1°F warmer than the previous record set in 1954.

Most of extreme southwestern Oklahoma has had 43 consecutive days or more with temperatures above 100°F, and the streak is expected to continue well into August. On average, this region expects 30-40 days above the century mark each year. According to Gary McManus, Oklahoma associate state climatologist, the highest number of 100°F days ever in one year is 86, set in Hollis in 1956:

http://www.climatecentral.org/news/records-fall-during-southern-great-plains-heat-wave/