Bare in mind, I've heard these exact same arguments for years, so it's difficult not to laugh at them.
"While Earth’s temperature has fluctuated...comparing today’s climate to periods like the Cambrian...ignores the real issue."
You're straw manning my argument. Of course, the earth was a molten ball of lava 6 billion years ago. Below is a link to a graph of 600 million years of earth's temps and atmospheric CO2. You should notice:
1) Global temps and CO2 concentrations are not even remotely correlated. Temps have gone up as CO2 concentration has gone down and vice versa.
2) Over 600 million years, the earth's global temps have only varied +/- 10C.
3) For most of the last 600 million years, CO2 has been at 3000ppm. And
4) Both global temps and atmospheric CO2 is at all time lows. Now in order for you to continue with your Doomsday position, you must first square these facts.
http://i.ibb.co/Rj3Vq18/600-million-years.png "We are in the Holocene epoch...a period that has been relatively stable,...since the Industrial Revolution...Earth’s average global temperature has risen...driven by fossil fuels and CO2 emissions."
Here is a graph of the temperature variations during the Holocene. Take note that the beginning of the Industrial Revolution in 1850 was also the end of the Little Ice Age. So, of course, temperatures have increased. The Middle Ages Warm Period, the Roman and Minoan Warm periods are all labeled. You should note that each one of these warm periods are warmer than the earth is today, and they also predate the invention of the internal combustion engine.
So for your anthropogenic climate hypothesis to be true, you first have to eliminate natural variation as a cause. Here is direct evidence that natural variation exists, it exists on a regular basis, it happens approximately every 1000 years, and we are scheduled for a global increase in temperatures right now. Unless you can rule out natural variation then you *cannot* claim human activity is the cause of the current warming since 1850.
http://i.ibb.co/KxDXfmS/Holocene.jpg And here is the same graph, with CO2. Notice, absolutely zero correlation between temperature and CO2.
http://i.ibb.co/NxgZm2X/Holocene-temps.png "Five of the six hottest years on record occurred between 2015 and 2020. The rapid warming we see today, driven by human activity, is far outside the natural variations of the Holocene."
Looking at the graphs above, would you like to retract this statement?
"The Medieval Warm Period (MWP) was a regional event"
The "it was regional" arguments are always fun. The wiki article on the WMP shows that it was detected globally.
http://en.wikipedia.org/wiki/Medieval_Warm_Period "Paleoclimatology studies using ice cores and tree rings show global temperatures during the MWP were much cooler than today."
You're referencing Michael Mann's work, who was the fool who produced the Hockey Stick graph. His research has been widely criticized for cherry picking data to produce a temperature history that deletes the MWP.
"You mock the 0.77 mm/year sea-level rise, implying it’s insignificant"
Are you really telling me that a less dense fluid like our atmosphere can significantly heat up a much more dense fluid like the ocean? LOL.
"Your claim that CO2 emissions don’t cause climate change is refuted by overwhelming scientific evidence."
CO2 emissions don't cause climate change once you reach the point of saturation. You could make it colder by reducing global CO2, but you'd have to reduce it below 220ppm. Keep in mind, if CO2 drops below 150ppm every plant on earth would die. Which leads us to the benefits of higher CO2. Plants need CO2. We are at near all time lows for atmospheric CO2. This is why greenhouses have to spend an enormous amount of money installing CO2 generators.
http://rimol.com/co2/co2-generator/ If the amount of atmospheric CO2 was indeed higher than ever, then greenhouses wouldn't need to do this, as plants would have evolved a photosynthesis process that was primed for lower CO2 concentrations then exist today. The fact that greenhouses have to go to the expense of doing this is direct evidence that we are currently in a CO2 starved atmosphere.
When plants have to struggle to take up CO2, they have to open their stomata wide, losing moisture to air. This further exacerbates droughts. When CO2 is higher, plants can open their stomata more narrowly, losing less moisture, which in turn keeps the ground moist. Here is a video of the same plant species growing at two different CO2 concentrations.
http://www.youtube.com/watch?v=32fNESgmzuI Furthermore, NASA's own research has shown that globally, the amount of plant foliage has increased significantly because of CO2 emissions. The increase in foliage is equal to the square mileage of the entire continental United States. Times 2. A greener world is a healthier world. We need more CO2, not less.
"A 2015 Nature study confirmed that increasing CO2 levels have directly increased downwelling longwave radiation at Earth’s surface, validating the greenhouse effect."
A study you didn't bother to link. But do you really think CO2 is the only greenhouse gas? The greenhouse gasses are CO2, methane, nitrous oxide, water vapor, and ozone. Yes, ozone is a greenhouse gas. As in the ozone layer. Since we banned CFC's the hole in the ozone layer has decreased, and atmospheric ozone has increased. So how do you know that increases to atmospheric ozone isn't causing climate change? Notice, this is precisely your argument, just using another gas. And I'll do ya one better. Ozone's IR absorption on a molecule by molecule basis is a stronger greenhouse gas than CO2, and furthermore, it absorbs IR at the same frequency of 15um that CO2 does. So how do you know that increases to atmospheric ozone isn't causing climate change?
http://webbook.nist.gov/cgi/inchi?ID=C10028156&Mask=80#IR-Spec By the way, I already know that ozone isn't contributing to climate change, but I'm not going to tell you why. I'll leave you to figure that out.
"long-term trend shows hurricane intensity increasing"
No, they're not gaining in intensity, and we've already established that the atmosphere cannot increase the temperature of the oceans. But notice the causal factor can go in reverse. If something like say THE SUN warmed the oceans, the ocean could in turn warm the atmosphere. You know, because thermal transfer happens more easily going from more dense things to less dense things.
"You argue that climate change is unfalsifiable because it accommodates opposite outcomes...This reflects a misunderstanding of how climate systems work."
No, YOU'RE the one who's not understanding the point of falsifiability. This is basic Philosophy of Science 101. So let's review. Suppose you make a claim that can not be tested. Like say, "chocolate is the best ice cream flavor". Because it cannot be tested to be true or false, then it is an unfalsifiable claim. You cannot establish what IS from what people like. This is an extension of Hume's IS/OUGHT problem.
When it comes to climate change, the question is not whether the earth's climate is a dynamic system that includes both droughts and floods, heat waves and blizzards. It obviously does include all these things and more. What makes anthropogenic climate change unfalsifiable is that ALL EVIDENCE, no matter what it is, can be used to justify the theory. This is why I asked you, what evidence would show that climate change is NOT happening? You need to pause and really consider the magnitude of that question.