Category Archives: global warming skeptics

Evidence for Global Warming: Is Global Warming Real?

Global warming is real, and it is caused by humans. How can I say this?

The evidence is overwhelming.

Global temperature records, kept by a variety of scientific organizations, show rising temperatures over the past 150 years. Snow cover across the northern hemisphere has declined over the past decades. Spring is coming earlier each year. The sea ice at the north pole is melting even faster than the climate models predict. The south pole also appears to be losing ice – again, faster than predicted.

Glaciers are melting all over the world, and animals are changing their behavior.

There are multiple streams of evidence pointing to greenhouse gases like CO2 as the cause of the current warming.

Global warming is real.

Is Global Warming Caused by Natural Cycles?

Global warming denialists love to assert that global warming is all natural. But is it? What’s the evidence?

Global Warming: Is It All Caused by Natural Cycles?

The American Denial of Global Warming

Here’s a good video, titled “The American Denial of Global Warming.” It gives a good overview of the science (and the history of the science) of global warming in its first half and the history of the denialist movement in its second half.

Here’s the thing: The science behind the greenhouse theory has been around since the 19th century. The fact is, we are seeing a warming planet, we have added enough CO2 to the environment to cause the warming, and we have known for more than a hundred years that this would be the case.

Hooray for Global Warming!

Increased CO2 and higher temperatures will be good for plants, so we’ll have more food – so global warming won’t be all bad, right?


Photo by Rene Schwietzke

This argument was first propagated by the Greening Earth Society, (see paper :
Forecasting World Food Supplies: The Impact of the Rising Atmospheric CO2 Concentration, Craig D. Idso and Keith E. Idso) and recently made a reappearance in the Competitive Enterprise Institute’s attempt to debunk Al Gore’s movie, An Inconvenient Truth.

I came across it again in the OISM’s 1998 paper about global warming that was linked to in a recent comment.

The truth is, we don’t know. A recent study suggests that rising CO2 could help plants grow in desert areas.

Many other studies suggest that CO2, in general, does have a positive effect on plant growth.

CO2 may give fast growing plants an edge (see here). Invasive plants might have an edge, but there appear to be limits:

(from Evidence of a feedback mechanism limiting plant response to elevated carbon dioxide, Nature 364, (1993), Diaz et al., emphasis mine)

In short-term experiments under productive laboratory conditions, native herbaceous plants differ widely in their potential to achieve higher yields at elevated concentrations of atmospheric carbon dioxide. The most responsive species appear to be large fast-growing perennials of recently disturbed fertile soils. These types of plants are currently increasing in abundance but it is not known whether this is an effect of rising carbon dioxide or is due to other factors. Doubts concerning the potential of natural vegetation for sustained response to rising carbon dioxide have arisen from experiments on infertile soils, where the stimulus to growth was curtailed by mineral nutrient limitations. Here we present evidence that mineral nutrient constraints on the fertilizer effect of elevated carbon dioxide can also occur on fertile soil and in the earliest stages of secondary succession. Our data indicate that there may be a feedback mechanism in which elevated carbon dioxide causes an increase in substrate release into the rhizosphere by non-mycorrhizal plants, leading to mineral nutrient sequestration by the expanded microflora and a consequent nutritional limitation on plant growth.

In plain English, different plants will respond differently to increased CO2. Many crops do not appear to grow better with more CO2 in the atmosphere.

One way to cope with global warming would be to use excess CO2 to boost the growth of certain plants, then burn those plants for fuel (Source).

Some further reading:

Food for Thought: Lower-Than-Expected Crop Yield Stimulation with Rising CO2 Concentrations, Long et al. science 312 June 2006


Climate change and world food supply

Rabett Run: Weeds grow well in high CO2. Crops?

Food-crop yields in future greenhouse-gas conditions lower than expected

EPA: Agriculture and Food Supply

Climate Change and Crop Yields: Beyond Cassandra, David Schimel: An analysis of recent data from a wide variety of field experiments suggests that previous studies overestimated the positive effects of higher carbon dioxide concentrations on crop yields.

Will there be enough Food?

Google Search for the paper by Idso and Idso, referred to above.

Will plants grow us out of trouble in a greenhouse world?

The build-up of CO2 in our atmosphere will be good!

Key Evidence for Global Warming Debunked?

If you spend a little time talking to global warming skeptics, you’ll run across some variation of this argument: the Mann Hockey Stick graph, which shows global temperatures trending dramatically upward, is flawed/faked/broken.

The so called “hockey stick” is a graph from a paper written in 1998 by Michael Mann and others (often referred to as MBH 98). It shows temperatures for the past 1000 years, with a dramatic upward swing in the 20th century. The graph attained a fair bit of notoriety when the International Panel on Climate Change featured it in its Third Assessment Report in 2001. Since then there have been several scientific papers published about it, endless websites and blog posts, and a congressional hearing, all on this single paper.

The general thrust of all this study, criticism, accusations of fraud, reworking, reexamining etc. is that there were indeed some flaws in the statistical methods used in MBH 98. However, multiple papers since then have indicated that the conclusions are valid.

Climate scientists have compared multiple datasets (called proxies) – ice samples, treerings, boreholes, historical documents, marine sediments – to get an idea of past climate variations. I’ll quote from the executive summary of the National Academy of Sciences “Surface Temperature Reconstructions for the Last 2000 Years”:

It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries….less confidence can be placed in large-scale surface temperature reconstructions for the period from AD 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since AD 900….Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about AD 900….


The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on icecaps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2000 years. Not all individual proxy records indicate that the recent warmth is unprecedented, although a larger fraction of geographically diverse sites experienced exceptional warmth during the late 20th century than during any other extended period from AD 900 onward.

The Wegman report, presented to Congress in 2005, was a detailed study of the statistical methods used in MBH 1998. Although it was widely hailed by climate change skeptics as a thorough debunking of Mann’s work, it is limited in scope. It’s worth some study if you want to understand the issue more fully, and a PDF can be found here. Commentary on the Wegman report here.

Other scientists have studied Mann’s conclusions and methods and found them to be robust. Some of these papers are summarized in the Wegman report.

Also take a look here.

I draw two conclusions from this:

1. There are lots of unknowns in the study of climate – both future predictions and past climate reconstructions.

2. There is substantial evidence that the current warming is unusual and goes beyond the ranges seen in the past 2000 years.

Also note that, whatever your opinion of the hockey stick graph itself, there is still strong evidence that the earth is warming and that humans are the cause.