Complex sentence: Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past, and these sources of information can be combined to find the climate model that best fits all the available data.
Split into simpler sentences: Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past. These data can be combined to find the climate model that best fits all the available data.