I'm a scientist. I've been watching the scientific literature on global climate change for decades. And I testify that there is a genuine consensus in the worldwide scientific community, and there has been a consensus since about 2003. We agree that:
You may have heard otherwise. There's a lot of misinformation going around.
Yes, I've done my homework. Yes, I'm sure. Yes, we're sure. No, we don't respect the skeptics. The accusations of fraud were investigated, thanks.
If you actually want to learn about this, I suggest avoiding TV and the press, and going to informed sources.
What we should do about it
It is not because I saw one killer piece of evidence, or saw a movie. It's because there are lots of different kinds of evidence, and they agree. This is an opinion that grew over time, as the facts piled up.
You've probably heard about melting glaciers, and about computer models of the climate. But you may not have heard that wildflowers in Britain are blooming earlier in the spring than they did a hundred years ago. There are birds nesting earlier in the spring. You may not have heard that mountaintop species in the California Sierras are living at higher elevations than before. You may not have heard about how weather patterns have increased the number of pine beetles and forest fires.
Some of this evidence is from field biologists - entomologists in Australia, for example. Some of it is reports of fishermen seeing changes in spawning grounds. Some of it is from NASA satellites, which measure the changes in gravitational pull as they pass over regions like Greenland. Some of it is from buoys floating in the ocean. Some of it is from the weather bureau, who report that the ratio of heat records to cold records has developed a major imbalance that didn't used to be there. Some of it is from geologists. Some of it is from geneticists, who've noticed recent genetic changes in a wildflower that increase its drought resistance. Some of it is the reports of how an increasingly hot ocean is killing coral reefs, and how an increasingly acidic ocean is killing shellfish. And then there's the report that higher wind speeds are reducing the average weight of albatrosses. Polar bears are weighing less, too. Some of the Adelie Penguin rookeries are a quarter the size that they were in the 1970's, because the food supply is decreasing.
In short, it's not just one thing, it's not just one group of people, it's not just because of some theory. The evidence comes from people who don't share a country, who don't share a political party, who don't share a wallet. Yet their reports in peer-reviewed journals create a coherent picture, and it's the picture that was predicted back in the 1980's.
That's why I'm sure.
It is plausible that the sea level will go up by five to nine meters. It's plausible because that happened normally, during the last interglacial, a hundred thousand years ago last Thursday. The worst case scenario is, well, worse. Frightening, even. But don't worry about it; major sea level rise isn't likely to get serious in your lifetime. And ocean acidification isn't going to kill off more than shellfish, at first. And the consequences of a few thousand species going extinct won't be felt right away.
What you should worry about is storms, drought, heat waves, and all the other extreme weather that a more-energetic atmosphere and warmer ocean will give. Average wind speeds are increasing. Plus, the jet stream is going to act differently when the Arctic Ocean becomes ice-free. That may already be having an effect on the USA. Another joker in the deck is that traditional power plants need cooling, and stop working in a major drought.
The planet has never seen this amount of CO2 arrive in this short a time, so in that sense we have already taken the planet's climate into unexplored territory.
Aside from worrying about your local area, you should worry about the world food supply. It's plausible that it will decrease in your lifetime. Yes, that would mean a decrease in world population. No, I am not Dr. Strangelove: there are people more blood-curdling than me, who already are talking about the details. It wouldn't be pretty. Let's not. Please.
The scientific community believes you should be skeptical when you hear something new. It's a good attitude. In the 80's, Global Warming was a bit speculative, and most of us were skeptical. In the 1990's, the evidence got better, and it became a minority opinion in its community. Then the evidence got even better, and it became a majority opinion. Around about 2003, we got good data about the temperature of the ocean, and the skeptical scientists pretty well conceded. The remaining skeptics are mostly outside the scientific community, and " ...are just as critical of one another's ideas as they are of conventional science." Incorrect conclusions like "global warming has paused in the last 5-15 years" usually involve cherry picking the data. I don't respect such people.
The skeptics who still claim fraud are pretty well conspiracy theorists. The accusations have resulted in at least six formal investigations, none of which found anything much. Most non-scientists don't know this, but ethics investigations happen all the time. The worldwide scientific community is about a million people, and in any given year, there will be at least one very high-profile investigation. I've seen these result in peer-reviewed journals issuing corrections or retractions. I've seen them result in someone being censured or fired. So, scientific ethics investigations are not just whitewashes. But none of the climate-science investigations resulted in censure. No climate-science papers had to be corrected. No censorship was found. The errors in the IPCC report turned out to be minor stuff 'way in the back pages. The "hockey stick" diagram was right.
In short, the accusations turned out to be junk. Which is what you'd expect, when you consider that every major scientific organization on the planet has endorsed modern climate science.
The milder accusations are that we are overstating the case, and that global warming will be something society can live with. Some say hopeful things, like "Carbon dioxide is plant food !" Unfortunately, world food prices are high lately and are not likely to drop. Plants that thrive in heat, storm, drought and flood are usually called "weeds", not "crops".
The press and media have been consistently terrible sources of information. In the name of "balance", they end every article by quoting someone from the Flat Earth Society. They also water down the warnings, and fail to connect dots.
RealClimate is the most reputable scientific access point. They link to the reputable FAQs, and give good analyses of the technical news. You can tell that it is run by scientists: they remember to be quantitative, to give uncertainties and caveats and references, and to point out what isn't known. This is exactly what the current "skeptics" do not do.
Climate Progress is a good resource for current news and political commentary. (It tends to non-Republican politics, because the Republican party has been a major force against any climate-related policies.) The guest bloggers vary in background, but they tend to be well-informed.
The bloggers at Weather Underground mix weather reporting with good climate information.
I subscribe to Science and Nature. The technical articles are tough sledding, but the news items are fairly good science journalism. Science magazines aimed at the general public (like Scientific American) are often good too.
Some people argue that action is pointless until all other countries decide to go along. I don't agree. For one thing, taking action will push renewable energy to be cheaper and better. We can collect carbon tax on the coal we ship to non-taxing countries. And we can collect carbon tax on imports in a way that levels the playing field with domestic products. We may not have the necessary political power, but our economic power will work better anyway.
Preventing a problem is always cheaper than fixing it afterwards. Please do not fool yourself into thinking that we should do something later, instead of now. For one thing, there may not be a good way to fix it afterwards.