Tackling climate change — and thus keeping the world inhabitable — is an achievable goal, but it will become prohibitively expensive if we wait to act. This is the key message from a leaked United Nations study that The New York Times reported on last week. Journalist Justin Gillis wrote about the risk of “severe economic disruption” and “wildly expensive” solutions — ones that may not even exist — if we don’t leverage existing technologies to shift the global economy away from carbon over the next 15 years.