How do we really know that, though? Let me also say I haven't personally done a lot research, so this is just how it comes across to me and I would like to learn more.
Man hasn't really been around long in the grand scheme of Earth. And educated man has been around for an even smaller amount of time. How do we know it's not supposed to be heating up? And what has changed so dramatically in the past few years that this is suddenly an issue?
Okay, well I'll explain how we know when I get to Malkovich cycles, but I am just going to through out all the normal natural variations in climate as well...
We know that this change isn't natural because the usual drivers of natural climate change have shown little to no warming trend since the 70's.
It's a well established fact that climate changes naturally. The question isn't "has climate changed in the past?" (of course it has) but "what is causing global warming now?" To answer that, we need to look at past drivers in climate change.
First, I'll talk about the sun. Solar variations have been the major driving factor for these earths climate in the past 10'000 or so years. We know that when sunspot activity has been low, it has caused little ice ages, and when it has been high it has caused warming (such as the medieval warming period). This tightly nit correlation, however, ended in 1975. This led a
team of scientists from Finland and Germany to conclude "during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source."
Next are the malkovich cycles. Earth's climate undergoes 120,000 year cycle of ice ages, broken by short warm periods called interglacials. Milankovitch cycles are changes in the Earth's orbit trigger an initial warming which warms the oceans and melts ice sheets - this releases CO2. The extra CO2 in the atmosphere causes further warming leading to interglacials ending the ice ages.
For the past 12,000 years, we've been in an interglacial. The current trend of the Milankovitch cycle is a gradual cooling down towards an ice age. But, for some reason were are warming, not cooling.
**Note Makovich cycles are also why carbon appears lag, not lead. In past warming cycles.
Lastly, there's global dimming. Global dimming is the phenomenon in which aerosols from things such as pollution, and mainly from volcanic eruptions, reflect the suns light from the atmosphere, causing a cooling effect. You can see that a series of volcanic eruptions in the 1800's caused a little ice age. These cooling periods from global dimming can last upwards of a decade.
The usual suspects in natural climate change (solar variations, volcanoes, Milankovitch cycles) have been absent over the past 3 decades of warming. This doesn't mean by itself that CO2 is the main cause of current global warming, you don't prove anthropogenic warming by eliminating all other options, but the primary causes of commonly cited climate change in the past have played little part in the current warming trend.
As for CO2,
empirical observations show that CO2 has a warming effect as a greenhouse gas, CO2 is increasing in the atmosphere and at the same time the increase in temperature that would be expected is happening. A theory suggesting that it is natural and not caused by CO2 would also need to explain why the expected (and observed) warming from CO2 has not stopped.
Wowie, that was a long post... but your not the first to inquire about natural change, so I thought I would just finally get it all out there!
