On the basis of geological evidence , it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere .
We present 3D global climate simulations of the early martian climate performed assuming a faint young sun and a CO _ { 2 } atmosphere with surface pressure between 0.1 and 7 bars .
The model includes a detailed representation of radiative transfer using revised CO _ { 2 } gas collision induced absorption properties , and a parameterisation of CO 2 ice cloud microphysical and radiative properties .
A wide range of possible climates is explored using various values of obliquities , orbital parameters , cloud microphysic parameters , atmospheric dust loading , and surface properties .

Unlike on present-day Mars , for pressures higher than a fraction of a bar surface temperatures vary with altitude because of adiabatic cooling / warming of the atmosphere .
In most simulations , CO _ { 2 } ice clouds cover a major part of the planet .
Previous studies suggested that they could have warmed the planet thanks to their scattering greenhouse effect .
However , even assuming parameters that maximize this effect , it does not exceed +15 K. Combined with the revised CO _ { 2 } spectroscopy and the impact of surface CO _ { 2 } ice on the planetary albedo , we find that a CO _ { 2 } atmosphere could not have raised the annual mean temperature above 0 ^ { \circ } C anywhere on the planet .
The collapse of the atmosphere into permanent CO _ { 2 } ice caps is predicted for pressures higher than 3 bar , or conversely at pressure lower than one bar if the obliquity is low enough .
Summertime diurnal mean surface temperatures above 0 ^ { \circ } C ( a condition which could have allowed rivers and lakes to form ) are predicted for obliquity larger than 40 ^ { \circ } at high latitudes but not in locations where most valley networks or layered sedimentary units are observed .
In the absence of other warming mechanisms , our climate model results are thus consistent with a cold early Mars scenario in which non climatic mechanisms must occur to explain the evidence for liquid water .
In a companion paper by Wordsworth et al. , we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail .