The phrase "cooling phase" refers to a period of time where something that was hot or heated begins to gradually become cooler. It describes the process of reducing or lowering temperature after an intense or heated activity. Full definition
I might add, with twenty different models, it's not hard to find one that expects a two year cooling phase. (dotearth.blogs.nytimes.com)
The empirical evidence continues to build within the climate science community that the world is experiencing some type of global cooling phase as a result of natural climate change forces. (c3headlines.com)
Of specific interest was how the waste form behaved both during the initial cooling phase and over the thousands of years the waste would be stored. (pnnl.gov)