Having worked in government agencies on climate change, and published on the issue, I would like to try to clarify some of the points raised in David Patterson’s letter of June 27 on the global warming ‘hiatus’.
The hiatus is an interval of slower rise in global temperatures, and many respected climate scientists agree that a hiatus has recently occurred. Virtually all climate scientists agree that natural variability (often as cycles) contributes to global temperatures, and some scientists think such variability may be the cause of the hiatus.
But Mr. Patterson’s letter may provoke climate change skeptics to say that the rise is due only to natural variability and not to man, or that any hiatus is proof of that. Mr. Patterson is right to support the scientific consensus that the measured rise in global temperatures since around 1850 is largely due to man; natural variability is a secondary contributor to this temperature record.
Recent papers suggest there may have been three hiatus intervals in the post-1850 record, the intervals starting around 1880, 1940 and 1998.
The cause may be a natural up-and-down cycle with its downturns starting at those times, but the temperature rise from man’s emissions of greenhouse gases almost overwhelms any such natural variability.
This is why the presence of a hiatus is often unclear, particularly over the last decade or two when the temperature rise from greenhouse emissions has been so marked.