I think most residential heat pumps use a closed system. A fluid (maybe water or anti-freeze) is circulated through loops of pipe in a well or a lake. The lake/well water never leaves, it just cools/warms the fluid in the loops. The heat pump uses that fluid to operate.
If it's against the law to use the lake for cooling/heating, many of the big cruisers on the lake are breaking the law. Many marine air conditioners pump lake water for cooling.
As to long term warming effects:
According to one website, the lake holds 625 billion gallons or about 5 trillion pounds. One BTU is required to raise a pound of water one degree. So 5 trillion BTU's would raise 5 trillion pounds one degree. An average home air conditioner is 10,000 BTU per hour. So it would take a million home air conditioners running for a 500 hours to heat the lake by one degree.
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