According to the paper, warmer tropical waters since the mid-18th century can be the result of both natural variability and human - driven climate changes. (sciencedaily.com)
So this effect could either be the result of natural variability in Earth's climate, or yet another effect of carbon dioxide and other greenhouse gases like water vapor trapping more heat and thus warming sea - surface temperatures. (scientificamerican.com)
This record also seems to show that the rise in methane levels in the last 10,000 years — thought by some to be a result of human agriculture — could simply be the result of natural variability in the decomposition of plants in boreal forests and wetlands. (scientificamerican.com)