Does Global warming occur periodicly?

[Plane]

http://www.discover.com/issues/oct-06/rd/sahara-desert-savanna-climate/



Although the Sahara has been bone-dry for most of its history, it undergoes an approximately 5,000-year humid period every 100,000 years as a result of variations in Earth's tilt and the shape of its orbit that change the way sunlight hits the planet. When that happens, it takes only a few centuries for the desert to become savanna, says Kröpelin. Global warming could make the next wet spell happen sooner than we think. "Since 1988 we have found evidence of increasing vegetation and rainfall," he says. "If we have five to six degrees of warming in the next centuries, evaporation on the oceans may turn the Sahara into a savanna, as it was 10,000 years ago."

[Amianthus]

Of course global warming happens periodically. All those intervals between ice ages were heralded in via global warming.

[_JS]

The question is not whether it occurs, but whether it occurs at such a quick rate and what impact industrialisation has had on that quick rate of warming.

Natural warming is well known, but rapid warming (or cooling) would be potentially much more devestating.

[Amianthus]

The question is not whether it occurs, but whether it occurs at such a quick rate and what impact industrialisation has had on that quick rate of warming.

History channel had a show on it not too long ago.

Scientists have documented several historical and pre-historical eras where there were 10 degree shifts in average temperatures (both up and down) in under a century. What we're looking at now is about a 4 degree shift in 2 centuries. So, yes, it has occured at "such a quick rate" in the past, without industry.

[_JS]

Interesting. Most of the temperature record reconstructions I've seen have not varied so greatly. We've seen about a one degree shift from 1910 to 2000, which is unique over the past two millennia as no such shift has taken place in such a short span of time. That even includes the Medieval warm up and the Little Ice Age.

From ice core samples from Greenland and Antarctica we know that the levels of certain pollutants have risen extremely quickly and to extreme rates over very recent time periods.

Can you explain how they narrow the data to within centuries for pre-human history? It was my understanding that when they get into geologic data on temperatures they tend to measure in millennia as anything smaller is typically unreliable.

[Amianthus]

Can you explain how they narrow the data to within centuries for pre-human history? It was my understanding that when they get into geologic data on temperatures they tend to measure in millennia as anything smaller is typically unreliable.

Ice cores can be used for stretches as short as 5 years. The Younger Dryas period, for example, shows a temperature change of 7 degrees C in 40-50 years, and it occured around 9,600 BC. This temperature change seems to be linked to shutdown of the North Atlantic thermohaline circulation, which we're also seeing some indications is occuring today.

Massive changes in ocean currents seems to affect the climate more than pollution (which, after all, comes from sources in nature as well as our pollutants). We're about due (past due, actually, IIRC) for a magnetic pole shift as well, which has also seemed to wreak havoc with the global environment historically.

[Amianthus]

We've seen about a one degree shift from 1910 to 2000, which is unique over the past two millennia as no such shift has taken place in such a short span of time. That even includes the Medieval warm up and the Little Ice Age.

I thought both the Medieval Climate Optimum and the Little Ice Age (which included the "Year Without a Summer") were both shifts of roughly 1 degree?

[_JS]

Hmmm.

I hadn't realised ice core isotopic temperature calculations were that precise. It wasn't that long ago that they changed some of the constants used in the formulae due to variations discovered in the relationships within glaciers, ocean water, and the temperature inside. Fascinating.

Paleoclimatology is amazing. There are graphs tracking the last 500 million years of climate and that is indeed how they ascertain the correlation between carbon dioxide and temperature.

I thought both the Medieval Climate Optimum and the Little Ice Age (which included the "Year Without a Summer") were both shifts of roughly 1 degree?

I'm using Celsius if that makes a difference, but the Little Ice Age was over a much longer period and so wasn't nearly as quick as today's global warming. Also, there is debate whether or not it was indeed a global phenomenon. It is interesting though, as the picture of Washington crossing the Deleware River amongst small ice chunks and pouring snow is likely a very rare sight today.

The same is true of the Medieval Optimum which was over a long period of time (3 to 4 centuries).

[Amianthus]

The same is true of the Medieval Optimum which was over a long period of time (3 to 4 centuries).

While the Medieval Optimum lasted for about 3 centuries, the time period for the shift itself was under one century. It took about 50-60 years to warm up, stayed that way about 150 years, then cooled off again in about 50-60 years. Ditto for the Little Ice Age (which just ended about the time of the US Civil War), though that cool period lasted longer than 150 years. We're only about 0.3 degrees C warmer than the Medieval Optimum right now. And this planet has been warmer still numerous times in the past.

I'd also like to note that most temperature comparisons for global warming are made against the "Little Ice Age" - which seems a bit specious to me.

[_JS]

We're only about 0.3 degrees C warmer than the Medieval Optimum right now. And this planet has been warmer still numerous times in the past.

But, the subsequent Carbon Dioxide levels that go along with the temperature increase are at much higher rates than in the past, which raises the question of how exactly those rates will be offset? I'm not betting the farm either way in this debate, but I don't believe that the scientists who study this are completely incompetent either.

[Amianthus]

But, the subsequent Carbon Dioxide levels that go along with the temperature increase are at much higher rates than in the past, which raises the question of how exactly those rates will be offset? I'm not betting the farm either way in this debate, but I don't believe that the scientists who study this are completely incompetent either.

Here is a graph of various data, including CO₂ - you'll notice similar, rapid spikes of atmospheric CO₂ several times over the last 420,000 years. Data was obtained from the Vostok core samples.

[Plane]

Do we really want to do something about Global Warming ?

Whether it is natural or not ?


Imagine that spreading disolved iron onto the central pacific would fertilise the alge there to metabolise ten times the CO2 that they now do , produceing a CO2 level something like it was in 1800 in a small time.

Would we want this to be done?




(No I am not certain that this method would work at all , but for the sake of argument lets assume that it would)