Final answer:
The last 18 years have not been cooler than previously due to decreased human activity.
Step-by-step explanation:
The statement 'The last 18 years have been cooler than previously because human activity has decreased' is false.
The best documented changes in Earth's climate are the great ice ages, which have periodically lowered the temperature of the Northern Hemisphere over the past half million years. The last ice age, which ended about 14,000 years ago, lasted for about 20,000 years. Since then, we have been in a warmer inter-glacial period. However, it is important to note that the current period of climate change, driven by human activities, is causing a rise in global temperatures.