Scientists have long been concerned that global warming may push Earth's climate system across a 'tipping point,' where rapid melting of ice and further warming may become irreversible -- a hotly debated scenario with an unclear picture of what this point of no return may look like. A new study suggests that combined warming of the North Atlantic and Pacific Oceans thousands of years ago may have provided the tipping point for abrupt warming and rapid melting of the northern ice sheets....