notes: The full passage from Kolbert's book (page 34) follows:
"'On one hand, you think its the earth's climate system; it's big, it's robust. And indeed, it has to be somewhat robust or else it would be changing all the time.' On the other hand, the climate record shows that it would be mistake to assume that change, when it comes, will come gradually. Perovich offered a comparison that he had heard from a glaciologist friend. The friend likened the climate system to a rowboat. 'You can tip and then you’ll just go back. You tip it and just go back. And then you tip it and you get to the other stable state, which is upside down.'"
In the following paragraph, Perovich offers the "tipping a boulder" metaphor.
[This rowboat metaphor is similar to the "Canting the Kayak" incident cited by glaciologist Richard Alley in Earth the Operators Manual, though Alley is using it to illustrate positive feedback, not a tipping point per se. He recalls a "certain dear person" who, as a first-time kayaker, when the kayak started to tip, leaned out to steady herself as she would on dry land. The she leaned in direction she was tipping, the more the kayak tipped.]
Source: Donald K. Perovich, research geophysicist at CRREL (Army Corps. of Engineers, Cold Regions Research & Engineering Laboratory), quoted in Elizabeth Kolberts, Field Notes from a Catastrophe, p. 34, sharing a metaphor he heard from a glaciologist friend.
image source: Boulder Boat Works