Thanks for the reply, gb - though I don't agree with all your points, I appreciate the time you took.

"But what is it about the idea of a current global warming trend that is being driven in significant part by human activity that you object to?

Do you deny that there is a greenhouse effect caused by atmospheric gases?"

Greenhouse affect is mostly caused by water vapor.

Agreement here.

"That carbon dioxide is one such gas?"

It is one of these greenhouse gases. But it's also fairly scace in your atmosphere: .038 % or 380 parts per million.
There far more water vapor in our atmosphere.

Of course. But that 0.038% is responsible for 9-26% of total greenhouse effect. Even at the lower figure, obviously a small increase in CO2 will have a large effect on greenhousing.

The surface temperature of the earth would be about -18 C if there were no greenhouse effect at all - it is currently about +15 C, or about 13% higher. A change of a couple of percent would be very significant.

"That much of the increase in atmospheric carbon dioxide in the past several decades is the result of burning fossil fuels and forests?"

It seems reasonable the some of the increase in atmosphere CO2 is due to these factors.

Mostly in agreement, though I think the evidence indicates that most of the increase is due to these factors.

"That current global average temperatures are increasing?"

In last couple centuries global temperatures are increasing- though if they weren't we would probably consider ourselves be still in the Little Ice age.

OK

"That there has been a sharp increase in the rate of temperature change in the last several decades?"

I would say it's an increase in global temperature which can be describe as part normal temperature fluctuation

I guess I can't convince you there, if the data doesn't.

"That this rate of increase is much greater than at any measureable time in the past? (note: rate of increase, not total increase)"

This would only true in sense that we don't have an accurate enough measurable past- that it can be considered accurate enough to measure tenths of degree in temperature.

I think the temperature scale is accurate enough, it's just that the time scale is too coarse for an accurate measure of rate of increase. But the average rate of increase where it can be measured is far less than the recent slope.

"That increasing average global temperatures will have a significant effect on both local and global climate patterns?"

Not really. It's more of case of local climate patterns overwhelming the barely measurable increases in Global temperatures.

There is more to climate than temperatures. Also, relatively small temperature increases can have large effects.

"That these effects will be harmful to many life forms (including humans) and ecosystems?"

No. If we had significant globe warming it would benefit life forms- and an increased CO2 is quite benefical to plants - and animals depending on plants to survive. I suppose plants that had slower growth could be affected because other plants around them are growing faster. Or plants surviving better in freezing conditions would have other plants crowding them out. But tropical areas are generally thought to not affected much according most models predicting effects from GW.

It might benefit some life forms, but it would harm far more. Good for mountain pine beetles, bad for mountain pine trees.

The US midwest could well become a desert, though the Canadian north could open up for agriculture - plus Canada would have an even greater percentage of North America's fresh water. That plus the longest undefended border in the world shared with the world's most powerful military makes me nervous.

That these effects will be harmful to the world economy?

No, and certainly not in comparison to political efforts like Kyoto.

I remember when car companies whined that increasing fuel economy would devastate them - Toyota and Honda seem to be doing OK these days. I think reducing CO2 emissions in an imaginative way could actually benefit the economy directly. Guess who wants us to keep burning oil, coal and natural gas??

"That there can be a "tipping point", at which gradual changes in climate could become rapid?"

Seems unlikely there is this tipping point. Earth has had much warmer periods than what we have now, or could have in the next century.

What does that have to do with the possibility of a tipping point?

"That if such a tipping point were passed, there could be significant melting of both the Greenland and Antarctic ice caps? (I don't think you would argue that sea levels would not increase significantly - in the range of many meters - were that antarctic cap to melt)."

The average temperature of Antarctic is -37 C. And Greenland is showing no signs of melting. And there is no reason to suspect that global temperature will likely rise, say 1 C in next 50 years. And if global temperature were to rise 2 C in next 50 years, Greenland's ice is not going to melt. If you took all the ice in Greenland and stacked in the middle America plains, it would take decades to melt. Or put in the Sahara Desert, it would still take decades.

All the ice doesn't have to melt for there to be a significant problem. If 10 percent were to melt, ocean levels would rise about 10 meters. Bye bye New York, Miami, LA, London, etc, etc. Even if there wasn't that much melting, increased temperatures could result in more rapid glacier flows and increased calving - if the ice is floating in the ocean, it would raise levels just as much as if it melted.

By the way, averages can be deceiving. The average voltage in your home wiring is zero - try sticking something in a light socket.

Even without ice cap melting and subsequent sea level rise, global warming will have catastrophic effects.