> However, climate models, based on the limited evidence to date of what the atmosphere was like, have struggled to simulate a Mars on which conditions were warm enough to allow lots of liquid water. Much of it would have been locked up in ice, they suggest.
Bit contradictory here.
The section you are quoting is titled 'climate conundrum'.
The writer is saying that it is visually obvious that there was running water at some point, but climate models do not support this.
That is why it is a .....conundrum.
Sept. 28, 2015 - We found water!
> These downhill flows, known as recurring slope lineae (RSL), often have been described as possibly related to liquid water. The new findings of hydrated salts on the slopes point to what that relationship may be to these dark features. The hydrated salts would lower the freezing point of a liquid brine, just as salt on roads here on Earth causes ice and snow to melt more rapidly. Scientists say it’s likely a shallow subsurface flow, with enough water wicking to the surface to explain the darkening. [1]
Aug. 23, 2016 - Maybe not...
> "Some type of water-related activity at the uphill end still might be a factor in triggering RSL, but the darkness of the ground is not associated with large amounts of water, either liquid or frozen," Edwards said. "Totally dry mechanisms for explaining RSL should not be ruled out." [2]
[1] https://www.nasa.gov/press-release/nasa-confirms-evidence-th...
[2] https://mars.jpl.nasa.gov/news/test-for-damp-ground-at-mars-...