NZJFor Home Search Join Author instructions NZIF website NZJFor Home NZJFor


New Zealand Journal of Forestry (2009) 54(3): 13–18
©New Zealand Institute of Forestry

Professional Paper
Reply to South, Brown and Dyck

Gareth Renowden 1

1 Gareth Renowden is a Canterbury-based writer and is the author of the book Hot Topic. Interested readers may contact him at

In this reply I shall attempt to answer some of the questions raised by South et al., and demonstrate that our current understanding of the climate system and where it is headed is quite sufficient to guide the creation of good public policy. A planet with no atmosphere orbiting the sun at the same distance as the Earth would have an average surface temperature of -18ēC. The effect of all the so-called greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide and various halocarbons) acting together is to provide 32ēC of warming, maintaining global average temperature at about +14ēC. Carbon dioxide plays a crucial role in that process, a fact that’s been understood since the first half of the 19th century, and the pioneering work of Fourier and Tyndall1. Work on past climate shows that during the ice ages that have dominated the planet’s climate over the last four million years, CO2 levels bottomed out at 180 ppm, while during the relatively brief warm interglacial periods it reached 280 to 300 ppm. 125,000 years ago during the last (Eemian) interglacial, CO2 levels peaked at 300 ppm, global temperatures were a degree or two warmer than at present, sea level was up to 5 metres higher, and there were hippos and crocodiles in the Thames. A recent paper2 suggests that the last time the planet experienced sustained CO2 levels of 400 ppm, 15 million years ago in the Miocene, global temperatures were 3-5ēC higher than now, and sea levels 25 - 40 m higher. At the time of writing, atmospheric CO2 has reached 387 ppm and is increasing at roughly 2 ppm per year. No hippos have been sighted off Westminster. Yet.
(no keywords)

Issues > 54(3) > Abstract

Download article as 1.2 MB PDF file

Free download.

(You can read PDF files with the free Adobe Acrobat Reader)