Back to website

1. Need for re-assessment of global warming science
Madhav L Khandekar
Letter submitted to Physics Today (28th June, 2004)

The present debate on Global Warming (GW) and the review of the book "The Discovery of Global Warming" appearing in Physics Today (June 2004) has prompted me to write this letter and make a case for re-assessment of the science of GW.

Several recent studies have seriously questioned many assumptions and observational evidence of GW that have been highlighted in the IPCC (Intergovernmental Panel on Climate Change) Climate Change documents (2001), as well as other scientific articles and news media reports. The Earth's mean temperature over the past millennium has been derived using proxy data (tree-rings, ice cores etc.) and is often presented by the well-known "Hockey Stick Curve" which shows an unprecedented warming of the earth's surface since about 1850 AD (Mann et al, 1999), the curve between 1850-2000 being shown in red representing the blade of the hockey stick, while the rest of the curve (hockey stick) shows mean temperature below a zero reference line. Recent studies (McIntyre &McKitrick, 2003: Esper et al, 2004) have questioned the methodology which produced the Hockey Stick Curve and have come up with revised earth's mean temperature variation which shows that during the MWP (Medieval Warm Period) of 8th thru 12th century, mean temperature was indeed warmer than the present. While the debate on this issue continues (McIntyre & McKitrick, 2004), more and more questions are being asked about whether the 20th century was indeed the warmest century in the last 1000 or more years.

A recent paper by McKitrick & Michaels (2004) documents that current temperature data may be significantly contaminated by extraneous factors like population growth, economic activity, GDP (Gross Domestic Product) etc. in many regions of the world. A paper by Kalnay and Cai (2003) estimates that urbanization may account for up to half the recent increase. A comprehensive paper by Pielke et al.(2002) suggests that urbanization and land-use change may be an important climate forcing and may overwhelm the GHG forcing in future climate projections. Thus the recent increase in Earth's mean temperature, estimated to be about 0.32C in 25 years, may not be all due to increase in atmospheric concentration of CO2 and other greenhouse gases(GHG).

The impact of GW on present and future climate on extreme weather events is being debated extensively. In a report prepared for the Government of Alberta (western Canada), I have carefully examined 20th century data (Khandekar, 2002) and have concluded that extreme weather events like thunderstorms/tornadoes, heat waves, winter blizzards etc. are NOT increasing anywhere in Canada at present and the probability of these events increasing in future remains very small. Many other reported studies, when carefully examined, show only a tenuous link between GW and extreme weather events.


Esper, J.,D.C.Frank and R.J.S.Wilson, 2004: Climate reconstructions: low-frequency ambition and high-frequency ratification. EOS, Vol.85,No.12,23 March 2004,p.113

Kalnay, E. and M.Cai,2003: Impact of urbanization and land-use change on climate. Nature, 423, p.528-531

Khandekar, M.L. 2002: Trends and changes in extreme weather events: An assessment with focus on Alberta and Canadian Prairies. Rept. Prepared for Alberta Environment, Edmonton, AB, 56p. ( available on:

Mann, M.E.,R.S.Bradley and M.K.Hughes,1999: Northern hemisphere temperatures during the past millennium: Inferences, uncertainties and limitations. Geoph. Res. Lettters, 26,759-762.

McIntyre, S. and R.McKitrick, 2003: Corrections to Mann et al proxy data base and northern hemisphere average temperature series. Energy & Environment, 14, 751-771.

----2004: Global temperature patterns and climate forcings over the past six centuries: a comment. Submitted to Nature

McKitrick, R. and P. Michaels, 2004: A test of corrections for extraneous signals in gridded surface temperature data. Climate Research, 26, 159-173.

Pielke, R.A.,Sr. et al. 2002: The influence of land-use change and landscape dynamics on the climate system: relevance to climate change policy beyond the radiative effect of greenhouse gases. Phil.Trans.Royal Society,London,A,360:1705-1719
Madhav L Khandekar, a Consulting Meteorologist, is a former Research Scientist from Environment Canada. He is on the Editorial Board of two international journals, Natural Hazards (Kluwer, Netherlands) and Climate Research (Inter-Research, Germany).