Does the CO2 Atmosphere of Mars Disprove Greenhouse Warming?

Does the CO2 atmosphere of Mars disprove greenhouse warming?

Check out this answer from Consensus:

The CO2 atmosphere of Mars does not disprove the concept of greenhouse warming but highlights the complexity of planetary climate systems. While CO2 alone may not have been sufficient to maintain warm conditions on early Mars, the interplay of various greenhouse gases and episodic atmospheric changes could have contributed to transient warm periods. Further research is needed to fully understand the mechanisms that could have supported liquid water and a warmer climate on early Mars.

The greenhouse effect is a critical component in understanding planetary climates, particularly in the context of early Mars. The presence of CO2 in the Martian atmosphere has been a focal point in debates about whether Mars could have supported liquid water and a warmer climate in its past. This article explores whether the CO2 atmosphere of Mars disproves the concept of greenhouse warming by examining various research findings.

CO2 and Early Martian Climate

Several studies have investigated the role of CO2 in maintaining a warm climate on early Mars. A one-dimensional, radiative-convective climate model suggests that CO2 clouds could significantly influence the greenhouse effect by altering the convective lapse rate and the planetary radiation budget. The condensation of CO2 decreases the lapse rate, thereby reducing the greenhouse effect’s magnitude, especially at low solar luminosities. This phenomenon might have precluded warm, globally averaged surface temperatures unless other greenhouse gases were present alongside CO2 and H2O1.

Episodic Greenhouse States

Research indicates that episodic pulses of CO2 injected into the Martian atmosphere could have created stable, higher pressure, warmer greenhouse states. These pulses, occurring more recently than 4 billion years ago, could have raised global mean temperatures above 240-250 K for extended periods, even accounting for CO2 condensation. This mechanism could have facilitated a limited hydrological cycle, contributing to fluvial erosion and valley formation on Mars2.

CO2 Snow and Atmospheric Dynamics

Experimental studies on CO2 nucleation and growth under Martian conditions reveal that CO2 clouds, depending on particle size, could lead to either warming or cooling. The formation of CO2 snow, characterized by a small number of large particles, suggests that CO2 clouds might not have been as effective in warming early Mars as previously thought3.

CO2 Inventory and Atmospheric Loss

Spacecraft measurements have provided insights into the CO2 inventory on Mars, indicating that significant amounts of CO2 have been lost to space through impact ejection and solar wind stripping. Additionally, CO2 has been sequestered in non-atmospheric reservoirs such as CO2 ice, clathrate hydrate, adsorbed gas, or carbonate minerals. These processes likely contributed to the transition from an early, warmer atmosphere to the current cold, dry state4.

Greenhouse Gases and Paleoclimate

The presence of liquid water on Mars in the past is supported by surface features such as valley networks. However, detailed radiation models suggest that even with a thick CO2 atmosphere, reduced solar luminosity early in the solar system’s history would have prevented sufficient greenhouse warming. The addition of other gases like H2O and SO2 also failed to raise surface temperatures above freezing, although they might have allowed for the formation of features by brines5.

The Case for a Wet, Warm Early Mars

Theoretical models support the idea that early Mars had a dense CO2 atmosphere and a warm, wet climate. CO2 pressures between 1 and 5 bars would have been necessary to keep surface temperatures above freezing. This dense atmosphere could have persisted for a geologically significant period if atmospheric CO2 was continuously resupplied, likely through volcanic activity6.

Does the CO2 atmosphere of Mars disprove greenhouse warming?

Joel Younger has answered Extremely Unlikely

An expert from University of Adelaide in Atmospheric Science, Astronomy, Meteorology, Physics, Plasma Physics

Short answer: The atmosphere of Mars is very thin, so while CO2 does convert infrared radiation into heat (faster moving gas molecules), there just isn’t much of it to heat up. Plus, Mars is farther away from the Sun than Earth, so it doesn’t get as much energy to warm it up.

Let’s start with some background on what the greenhouse effect is. If a planet has no atmosphere, we can easily calculate the temperature of a planet’s surface based on the amount of energy the Sun produces, the distance of the planet from the Sun, and the amount of light that the planet reflects. This is known as the effective temperature. To figure out what the temperature of a planet with an atmosphere is, we need to think about how much solar radiation is absorbed going down through the atmosphere, and how much radiation coming off of the planet’s surface is absorbed by the atmosphere before it gets to space. The greenhouse effect is when some of the energy radiated upward by a planet’s surface is absorbed and converted to heat before it gets to space.

The Sun emits light at many frequencies, but most of the energy is in the visible part of the spectrum that we can see with our eyes. On Earth, UV is absorbed by ozone in the stratosphere, which heats the middle atmospere, but Earth’s atmosphere is almost completely transparent to visible light. The ground, however absorbs some of this light, is heated, and radiates energy away at wavelengths based on the surface temperature, which is mainly infrared (IR) light for terrestrial temperatures. IR gets absorbed by “greenhouse” gases like CO2, water vapor, and methane, which heats up the absorbing gases. Atmospheric scientists refer to this process as the radiative balance or radiative equilibrium of downward shortwave (visible light) and upward longwave (infrared) radiation. The mathematics can get complicated, but the general idea is: radiation going down from the Sun or up from the ground can be absorbed, which heats the atmosphere.

The reason the martian surface is cold (compared to Earth) is twofold:

1. The thing about the atmosphere of Mars is that there isn’t very much of it. Compare the density of the atmosphere at the surface for Earth: 1.27 kg/m3 and Mars: 0.020 kg/m3. There is much less atmospheric mass between the surface of Mars and space than there is on Earth. Mars’s atmosphere is what is known as “optically thin”, which means that the probability of an infrared photon being absorbed as it travels up or down through the atmosphere is very low. Earth, on the other hand, has much more atmosphere between the surface and space, making it much more likely that infrared radiation from the surface will be absorbed by the atmosphere. The calculated effective temperature of the surface of Mars is -63 C, which is also the average measured surface temperature. This means that the atmosphere is so thin, it absorbs negligible amounts of IR radiation coming off of the ground.
2. Mars is further away from the Sun than Earth, which means it receives much less energy from the Sun. Mars orbits at more than 1.5 times the distance that Earth does from the Sun, so it only gets about 43% of the energy that Earth does (586 W/m2 for Mars vs. 1361 W/m2 for Earth).

Numbers used here were taken from the National Space Science Data Center at https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html

More information on radiative equilibrium (including the grey atmosphere approximation not discussed here) in planetary atmospheres can be found in texts like The Physics of Atmospheres by John Houghton or An Introduction to Atmospheric Physics by David Andrews.

Does the CO2 atmosphere of Mars disprove greenhouse warming?

Jan Cami has answered Extremely Unlikely

An expert from University of Western Ontario in Astronomy, Astrophysics, Astrochemistry, Physics

The greenhouse effect is a fairly well understood and tested phenomenon that you can easily recreate in a laboratory setting. CO2 is a greenhouse gas, and so any planet with CO2 in the atmosphere will experience some greenhouse warming as a consequence. The Earth, Venus, and Mars all have CO2 in their atmospheres, and thus they all experience the greenhouse effect to some extent. However, the magnitude of the greenhouse effect between those three planets is wildly diffferent. Several factors influence the amount of greenhouse warming: the distance from the Sun (the further away, the less radiation is received), the reflectivity of the planet (if more radiation is reflected, less can be used for heating), how much atmosphere there is, and the concentration of greenhouse gases. It is true that Mars has a much higher fraction of CO2 in its atmosphere than the Earth: for Mars, about 95% of the atmosphere is CO2, whereas for the Earth it is far less than 1%. However, Mars simply has far less of an atmosphere than the Earth. The surface pressure due to the atmosphere on Earth is 1 bar, but on Mars it is less than 1/100 of that! That makes the greenhouse warming much smaller on Mars than on the Earth — but it is still there! For the Earth, the greenhouse effect makes the planet on average about 30 degrees Celsius warmer than it would be without the greenhouse effect; for Mars, the greenhouse warming is only about 6 degrees. Venus is another illustration. Venus has an atmosphere that is very similar to Mars in terms of relative composition: about 96% of the atmosphere is CO2. But Venus has much more atmosphere than the Earth: at the surface, pressure is 90 bars! Consequently, the greenhouse effect is much more pronounced on Venus: the greenhouse warming is more than 500 degrees — which is why on the surface of Venus, it’s hot enough to melt lead. So all planets with CO2 in their atmospheres experience greenhouse warming; how much warming precisely depends also on how much atmosphere they have, and how far away from the Sun they are.

Does the CO2 atmosphere of Mars disprove greenhouse warming?

Francois Forget has answered Extremely Unlikely

An expert from Centre National de Recherche Scientifique in Planetology

On the opposite the study of Mars confirms the reality of greenhouse warming.

The same radiative transfer calculations that are used on the Earth to demonstrate greenhouse warming can be performed to the thin CO2 Mars atmosphere and they explain and match very well the observations of atmospheric and surface temperatures on Mars.