Which sentence shows a flaw or mistake in logic?

a volcanic eruption in the early 1800s threw so much ash into the air that it caused a global temperature dip.
sunspot activity affects earth's climate, which means that human activity is not a factor.
ice reflects more heat away from the earth than open water, which absorbs heat when it is exposed to the sun.
ocean currents, such as the gulf stream and the labrador current, have a significant effect on local climates.