The Real Signs of Human-Caused Global Warming

Human being are causing a rapid increase of greenhouse gases in the atmosphere, with carbon dioxide and methane gases being the most well known.

But how have increasing greenhouse gas influenced the weather and climate we experience today?  What current effects are unequivocal, right now?

The media is full of claims regarding the current impacts of increasing greenhouse gases, but many of these claims are without a good basis in science.    Some recent examples include:

• That Hurricane Harvey's heavy rains were a sign of global warming.
• That the recent California wildfires were the result of human-caused warming.
• That winter storms in the Northwest are becoming more intense due to global warming
• That global warming is causing more droughts in the West.
• That increasing greenhouse gases are causing more cold waves over the eastern U.S.

And there are many, more.  A weakness of many of media and activist claims is that one extreme event can not provide "proof" of human-caused global warming.   Only statistically significant trends have meaning when one is talking about climate change.  And many of the claims are inconsistent with climate models and basic statistics.

But me be clear: increasing greenhouse gases associated with human activities are changing our climate  in profound ways, and there are "fingerprints" of human effects today that provide compelling evidence of human influence.

When one wants to identify a thief, finding their fingerprints often provide incontrovertible proof of their guilt.   What are some "fingerprints" of increasing greenhouse gases on the weather/climate system?

1.  Fingerprint 1:  Greater warmer in the Arctic than the midlatitudes.

When climate models are run with increased greenhouse gases, virtually all of them not only show warming, but they show a similar distribution of warming.  Below is an an example, showing an average for several global climate models of the change in surface temperature during this century assuming that greenhouse gases keep up increasing at the current pace.

 Blue is cooling, red is warming.  All warming.  Note that the Arctic warms more than anywhere else, and the continents warm up more than the oceans.

Change in average surface temperature (a) and change in average precipitation (b) based on multi-model mean projections for 2081–2100 relative to 1986-2005 under the RCP2.6 (left) and RCP8.5 (right) scenarios. The number of models used to calculate the multi-model mean is indicated in the upper right corner of each panel. Stippling (i.e., dots) shows regions where the projected change is large compared to natural internal variability, and where at least 90% of models agree on the sign of change. Hatching (i.e., diagonal lines) shows regions where the projected change is less than one standard deviation of the natural internal variability

The greater Arctic warming is a major fingerprint of increasing greenhouse gases, one that is reflected in the observed changes in temperature during the last 70 years (see map below).  There are a number of reasons that the Arctic warms faster, including the loss of reflective sea ice and some subtle radiative effects, I won't get into.

A plot of the temperature change over the same period, but averaged by latitude clearly shows the Arctic warming.

2.  Stratospheric Cooling While the Lower Atmosphere Warms

Perhaps the most compelling support for the influence for increasing greenhouse gases might come as a surprise to many:  the cooling of the stratosphere.   Increasing CO2 warms the lower atmosphere through a process that is analogous to how a blanket warms you when sleeping...by slowing the loss of heat away from your body.  But having an efficient emitter of infrared radiation aloft actually results in cooling of the upper atmosphere.   So warming below and cooling aloft is a potent fingerprint of the influence of CO2.

Observations in the stratosphere and in the layers above (e.g., mesosphere, thermosphere), shown below and in the literature, demonstrate the cooling trend.  In the first figure, the blue and purple colors show cooling between 1979 and 2012 in the lower stratosphere, while the second figure shows cooling in the layers above the stratosphere (which ends about 50 km above the surface).

The cooling above the middle stratosphere is very important, because another issue (the weakening of the ozone layer from human emitted freon and other chlorfluoromethanes) could have produced some cooling.  But not in the layers above.

3.  Warming of the Earth's Atmosphere and Oceans and Model Experiments

As described in many places, the general temperature change of the past century determined by all the major centers are very similar, with cool temperatures during the late 19th century, warming during the first half of the 29th century, a leveling off from roughly 1950 to 1975, warming during the late 20th century, and a leveling off during the past 15 years.  The warming has been about 1C during the the entire period.  Analyses of the impacts of human emitted greenhouse gases suggest that our influence on warming would be mainly significant after 1970.

At first glance, this warming is not a compelling argument for human influence, since there was warming during the early part of the 20th century, which was probably natural.  There is some research that suggests that the pause during the middle of the 20th century might be associated with human-produced particles that scattered solar radiation back to space. 

As I noted before, atmospheric scientists run climate models to simulate the evolution of the earth's atmosphere. If we run the model with only natural forcing (e.g., volcanoes, keeping greenhouse gases constant at pre-industrial values), the simulations do ok until about 1970,  but are way too cool during the past 40 years (see top figure below, the black line is the observed global temperature, the red and green lines are the average of collections of climate models).    But add changing greenhouse gases and the models are much closer to the truth (although they are a bit too warm during the last decade).

This is strong evidence of the important of human-produced greenhouse gases during the last half century.    Not as compelling perhaps as my first two fingerprints, but together with them, a pretty strong argument that humans are changing the climate.

The media and many politicians have not given enough emphasis to the above fingerprints, which are really the best evidence we have for human-caused global warming.  Instead they have pushed much weaker "proofs", such as a few big tropical storm events (e. g., Katrina, Sandy, Harvey) and the Washington/California wildfires.  As described in a excellent analysis by NOAA's GFDL, the connection of current contemporary hurricanes  with global warming is a weak one.  And the increase in western wildfires reflects in part the

mismanagement of our forests, suppression of natural fires for 70 years, the spread of invasive cheatgrass, and huge increase of people living and recreating in the forest environment.

In short, there is plenty of strong evidence of human impacts on climate change, it just takes a little study to appreciate why they are compelling and important.




from Cliff Mass Weather and Climate Blog http://ift.tt/2yZRdVb