WASHINGTON — Man-made global warming may have started a few decades earlier than scientists previously figured, a new study suggests. Instead of the late 1800s, a slight almost imperceptible warming can now be tracked to around 1850 in North America, Europe and Asia, according to a new study based on coral, microscopic organisms, ice cores, cave samples, tree rings and computer simulations. And that happened when heat-trapping gases from burning fossil fuels were tiny compared with now, which means “the speed at which the climate responds to even a small change in greenhouse gases appears to be quite fast,” said study lead author Nerilie Abram, a paleoclimate scientist at the Australian National University.