Global Climate Change

How Do We Know the Planet is Warming?

That the earth is warming is a concept that has become scientific consensus. This means that it's understood to be true by an overwhelming majority of scientists. Moreover, many scientists have come to the conclusion that humans have caused that warming due, in large part, to the Industrial Revolution and the energy and sustenance needs of a growing population. To a person who knows nothing about global warming, it must be really confusing and easy to dismiss as 'alarmism'. Some of it is alarming when you understand how complex the problem has become.

Hopefully I can help summarize some of what we know and why the majority of scientists are coming to the conclusion that humans have created this problem. New studies are coming out every other day, so this page could quickly become outdated although the core goal is to cover the basics. Those details are less likely to change, as some parts are century-old knowledge.

The Cause of Global Warming: The Greenhouse Effect

A greenhouse is designed with glass in order to allow sunlight and heat through, and prevent this same heat from escaping. Greenhouse gases are the 'glass' and Earth is the 'greenhouse'. With rising carbon dioxide, methane, and other greenhouse gases in the atmosphere, Earth will warm. See more on the Greenhouse Effect to learn about the various gases at fault, including water vapor and its positive feedback loop.

Measureable Effects of Climate Change

Ocean Absorption of CO2 and Acidification

Global warming produces a few measureable effects that can cause changes on a global scale. The ocean absorbs some CO2 in a process called air-sea gas exchange. This prevents warming from happening so quickly, as the gas is not stuck in the air. This is why I'd mention it first. This is part of a process known as the carbon cycle. The planet will balance out through this natural mechanism, but the ocean can only absorb so much CO2. It will eventually release it back into the air, regardless - that's the natural process. We're producing far too much for it to handle, thus it's becoming acidic through a chemical process. The uptake of carbon also makes climate models difficult to predict, though it is protective and has helped to stop things from escalating too quickly for us to make changes.

Warming Surface and Ocean Temperatures

The second effect is, of course, warming of the ocean, air, and land. Observe the video from NASA at the top of this page, which starts around ~1885, well after the industrial revolution. Note what happens around the 1940s-1950s when industry boomed like never before after the war. It begins to warm much more rapidly. These are trends taken from temperature data from around the globe, so sometimes of course it's a bit cooler in an area, but the trend is upward. It's easy to see that things have escalated as the population grew and industry expanded. Worse, 2016 has been the hottest year on record based on average temperature. For the record, the average global temperature is around 57F or 14C (source). The average temperature was 1C higher than the previous year, 2016 is worse by a bit as you can see in the first link. There was an El Nino this year, which contributed to that increase. La Nina should reduce the global temperature for a time. Of concern is the average over time, and if an El Nino could outpace all previous temperature records at the current level of CO2, major changes could happen the next time this cycle repeats itself. Also of interest for the immediate future, is what the minimum temperature will reach with La Nina.

Ice Melt

Glaciers, snow and ice are retreating in many areas of the planet. They go through cycles: each summer, a portion of the ice melts. It's extended once again during the winter. When ice is not replaced as fast as it's lost, it's said to be in retreat. Some of this is easy to measure with simple satellite data. We can also see it happening with our own eyes, as massive glaciers break apart. Temperature measurement shows us the Earth is warming, while ice retreat gives us powerful evidence that it is happening. Thankfully we have maps to look to from the past to know just how bad it's become. The Greenland Ice Sheet is losing about 200 cubic kilometers of ice per year (source/wiki). Some scientists fear that multiple factors could come together to increase melt much faster than the current rate.

Rising Sea Levels

As ocean and sea-level temperatures rise, ice melts. It's still cold at the poles, but the water is warmer and of course during Summer each of them have opportunities to melt a bit more. Additionally, as sea water warms as it expands. While this expansion is not large, we're talking about Oceans, which make up the greater portion of the surface of Earth. This results in the most well-known effect of climate change, rising sea level. There is a massive volume of ice in the Arctic (which is not a continent, it's ice!), including the Greenland Ice Sheet, and some parts of Antarctica are ice shelves.

The "97% of Scientists Consensus"

This gets thrown around a lot as a strong point for the argument that humans caused global warming. Scientists studied the research of other scientists to determine if their papers/articles supported global warming and that humans are causing climate change and came to this huge number. Here is the study in question.

Most scientists will not disagree with the concept that humanity is contributing. The question is, "to what extent". It's well established that adding greenhouse gases to the atmosphere will increase the planet's temperature. We learned that 100 years ago! However, the question people ask is if humans really contributed to it or if we're in some kind of 'solar' cycle, with the sun blasting rays at the planet at a higher rate. The sun does go through cycles of around 11 years, but those cycles only cause a 1/3 of a fahrenheight increase in temperature (source), much smaller than the effects of CO2. These specific cycles play a very small role in warming on Earth, although other types of non-periodic solar changes can occur.

Plenty of people - scientists too - have tried to debunk this paper, saying that the authors had low standards for adding a paper to the study - that they only had to have two criteria, that humans contributed to global warming and that greenhouse gases raise the temperature of the planet. I suppose for someone to disagree with this based on those criteria being required, their question is if humans have anything to do with the current warming trend.

The most recent study's authors did an Ask Me Anything (AMA) on Reddit that you can view here. I mention the attempts to debunk it, and the authors do address that in the AMA.

Further Reading
Svante Arrhenius
Guy Stewart Callendar
Roger Revelle
Gilbert Plass