The phrase "steeper warming" refers to the increasing rate or speed at which the temperature is rising. It means that the temperature is increasing more quickly over time. Full definition
Interglacial trends over the past 400,000 years exhibit steep warming onsets, slower cooling rates and nearly flat plateaus. (co2coalition.org)
Currently, instrumental temperature characteristics are consistent with natural climate variability of short - term events and steep warming trendlines. (co2coalition.org)
But for his study, Muller took the objective road and found that there is indeed «very steep warming.» (blogs.sierraclub.org)