How Do We Know That We Are Causing Global Warming?

The vast majority of people will not become climate scientists, science is hard and even people that do it for a living don't understand absolutely everything, and thus we're faced with the problem of not being able to all understand everything about climate science. Why then should the average person trust that global warming is occurring and leading to changes in our climate? I've compiled my favorite lines of evidence for it here.

I don't intend for this to be an exhaustive scientific analysis of all of the evidence as this is a blog post. The whole point is to filter through to some very basic arguments that are hopefully easy to understand by anyone and paint a reasonably compelling picture. I'll try to include a respected source for every non-speculative and/or non-obvious claim if you want to read further. Major agencies have much more comprehensive collections of evidence and I will list some at the end of this post.

How Do We Know The Earth Is Warming?

  1. Numerous, independent groups using independent measurement systems have arrived at the conclusion that the Earth is warming and has been for over a century. Five very well respected groups are:
  2. Things you would expect to happen assuming that the Earth is warming are happening. Some examples:
  3. The agreement between warming measurements between groups is very good: https://www.carbonbrief.org/factcheck-mail-sundays-astonishing-evidence-global-temperature-rise
How Do We Know Humans Are Contributors To The Warming?

  1. We know that greenhouse gases exist and how they function: https://en.wikipedia.org/wiki/Greenhouse_gas
  2. We know that greenhouse gas concentrations in the atmosphere are rising over time: 
  3. We know that this increase over time is absorbing heat: http://phys.org/news/2015-02-carbon-dioxide-greenhouse-effect.html
  4. We know that the ocean is absorbing more carbon: http://www.pmel.noaa.gov/co2/story/Ocean+Acidification
  5. We know how much humans are emitting: https://www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data
  6. With carbon at least, we know that it is coming from very old sources. Given that we are currently pumping large amounts of very old carbon into the atmosphere through the burning of fossil fuels, this points to humans contributing to the increase in greenhouse gas concentrations and thus an increase in global warming: 
How Do We Know It's Not Something Else?

When we add up contributions from all other sources it doesn't match what we observe, but when we add in human emissions then it matches quite well.
  1. This Bloomberg article has a nice visualization of NASA data: https://www.bloomberg.com/graphics/2015-whats-warming-the-world/
  2. Union of Concerned Scientists has a good article summarizing it here: http://www.ucsusa.org/global_warming/science_and_impacts/science/human-contribution-to-gw-faq.html#.WGPuhhsrJEY
How Do We Know It Will Continue To Warm And How Much Will It Warm?

This part gets harder. The basic physics of greenhouse gases combined with the lines of evidence above lead to the ability to make models. A number of issues make modeling difficult and three of those are:
  1. We don't know exactly how sensitive the Earth is to CO2 levels: https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-6.html
  2. It's hard to anticipate the effects of all of the feedback mechanisms. For example...ice reflects incoming sunlight and cools the Earth. Higher temperatures melt ice. Thus, higher temperatures lead to more warming. This is a positive feedback and there are many of these that are difficult to model.
  3. We don't know exactly what our emissions will be going forward. Will solar, wind, etc., dominate? There are several scenarios assumed in models and they are usually denoted by the acronym RCP (Representative Concentration Pathways).
Many agencies around the world attempt to model the future climate based on everything we know, and these are updated as new data come in. A previous post here shows the extremes in the standard (CMIP5) models that I have used primarily on this site.

That being said, models have proven fairly reliable and there are a number of decent analyses of this out there. Two examples are:
As a note, this uncertainty is why when you see reputable articles about climate change, avoiding climate disaster, etc., they'll often contain wording like '<33% chance of blah blah blah'. The IPCC actually defined ranges and used words to represent them (e.g., 'very likely' = 90-99% chance).



Summary

We know then that:
  • The Earth is warming.
  • We are emitting a lot of greenhouse gases, primarily through fossil fuel usage (also through other things like farming).
  • The greenhouse gases we are emitting are warming the Earth and at least some of the greenhouse gases that we know are warming the Earth come from human sources.
  • We know other contributors (sun, volcanoes, etc.) do not explain what we observe.
  • We can model how this will continue going forward and our models while not perfect are still reliable and definitely better than nothing.
And just in case you would prefer to trust authorities (I know this argument is a fallacy but we're lying to ourselves if we claim we don't do this...none of us have time to learn everything):
More in depth explanations of much of this can be found here (I'm not putting the list here because I don't want to have to update it in multiple places when I find new collections of data that I like):

 www.cityprojections.com/reading.html


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