I have some simple observations to point out.
The human population is rising, and it is rising faster all the time. I don't think I need to provide supporting references for this; it should be common knowledge.
Heat waves are becoming more common. "This hot extreme, which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area." This is happening globally. The hot spots are not always in the same place; sometimes it will be us, sometimes Europe, China, and/or Russia (notably in 2010, which had some fallout starting with food riots in Tunisia in 2011). The area they cover is on an upward trend globally.
The heat wave and drought that the midwest experienced last year lowered yields per acre. Not all Kansas counties were affected equally, but "Statewide, the corn yield averaged 96 bushels per acre, down 11 bushels from 2011 and the lowest Kansas corn yield since 1983." The drought and heat were worse in the west than they were in the east.
Farmers are not stupid. They have done a very good job of optimizing food production. They have first selected the lands best suited for agricultural production, and only as demand has grown have they expanded into less productive acreage. Not only are highly unusual events becoming more common, but climate zones are shifting in general. The combination of these changing factors is going to make yields per acre become less optimized, and less optimized production implies either less product, or higher prices for the same amount of product.
Since we live on a finite earth, rising population would be a problem at some point regardless. What I'm saying is that it would be less painful to deal with an increasing population if our ability to grow food were not decreasing at the same time. The least we can do is quit pretending we don't have a problem; that's just deluding ourselves. Some people are called alarmists, I call them them realists. On our current path, our future is "Cloudy with a chance of war".
In 1824, Joseph Fourier found that the earth was considerably warmer than it shoud be based on the work on thermodynamics (the study of energy exchanges) that he was conducting. If you are trained in mathematics or physics, you might recognize Fourier as the same person who gave us Fourier Analysis, and many other clever bits of science and math. In a nutshell, bodies radiant energy at a rate dependent on their temperature, and if you know how much energy is inbound (sunlight), you can calculate a temperature where energy outbound equals energy inbound. Earth was too warm, and Fourier deduced that Earth's atmosphere acted as a sort of insulation.
John Tyndall was also working on radiative energy transfer, a bit later in the 1850s. He also contributed many things that are now basic coursework in science classes, but the most relevant for the current topic is that he discovered that carbon dioxide absorbed and emitted radiation in the same part of the infrared spectrum as Earth emits. He was able to isolate the effect that various gases in the atmosphere have on radiated heat energy. This particular work was published in "On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connexion of Radiation, Absorption, and Conduction".
Svante Arrhenius was the first person to make the connection that human activities (mostly the burning of coal) increasing the carbon dioxide content of the atmosphere might, eventually, cause Earth to get warmer. Arrhenius was a pretty sharp guy as well; he taught himself to read at the age of three and, later of course, won the Nobel prize in chemistry, among other things. In 1896 he attempted to describe mathematically how much of an effect additional carbon dioxide should have. He incorporated his own work and built upon the earlier work of Fourier, Tyndall, Jožef Stefan, and Ludwig Boltzmann. (Those with a science or engineering background may be familiar with the Stefan-Boltzmann Law.) He did not have the refinement of knowing as much about feedback mechanisms, orbital mechanics, et cetera, as we do now, but his equation relating the content of carbon dioxide to the mean temperature of our planet serves as a first-order approximation and is still in use today.
In 1896, Arrhenius was a voice in the wilderness, and his idea was met with much derision at the time. Some decades later, in 1938, Guy Stewart Callendar revived and expanded upon the work of Arrhenius. He noted that industrial emissions of CO2 were already far greater than they were in Arrhenius' time, and that, even if the CO2 effect was pretty much saturated at sea level, the effect was a function of the density of the gas, and the density decreases with altitude. So, increasing the density overall rises the altitude at which it is not saturated, and that itself would increase the insulating effect and, therefore, the heat content would increase. Callendar was also met with debate, but this time the argument persisted. By the 1960s or so, the idea that people were impacting our planet's climate, and that CO2 was a prime driver, had won the debate, at least among scientists.
Charles Keeling was concerned about what might be happening to our planet, and in 1958 started measuring CO2 content in the middle of the ocean, far from industrial activities, and at the highest practical altitude in order to minimize the impact of daily cycles of plant growth on the measurements. Since 1958, there has been a continuous record of atmospheric CO2 content coming from the Mauna Loa Observatory on the big island of Hawaii. Many sites have been added to the record since then. They all tell the same story of an accelerating increase in CO2.
There are half a dozen or so institutions which have constructed records estimating the earth's surface temperature. The historical records are not nearly as complete as one would like to have for this undertaking, and the various institutions have used different math methods to minimise the effects of instrument bias, incomplete records, and the like. These different methodologies have yielded slightly different estimates for the mean temperature and rate of increase, but all the results show a marked and accelerating increase starting from the time that the industrial revolution started.
Recently, the concentration of CO2 in the atmosphere crossed 400 parts per million by volume (ppmv). This is not a magical number; it is not meaningfully different from 399ppmv or 401ppmv. However, people tend to gravitate to round numbers and this one has made the news lately. The last time CO2 levels were this high, about 3 million years ago, the planet had a very different climate. The continental ice sheets either did not exist or were much smaller than they are now, our ancestors were loosing body hair and learning to walk upright in the savannas of Africa, and pretty much none of the regions of earth had the same temperature and rain patterns that they do now. If you go back in time further, you will find the paleocene–eocene thermal maximum (PETM), another period of rapid CO2 increase and rapid temperature rise. The change in climate (and ocean pH) then was enough to destabilize ecosystems and cause the extinction of many ocean and mammal species. Rapid is a relative term; the PETM rise in CO2 and temperature took place over 20,000 years; we are on a path to cause the same changes in 200 years.
In summary, the basic foundations of the science related to global warming were laid down starting nearly 200 years ago. The prediction, based on science, that our activities would cause global warming was made about 100 years ago, and since then the planet has warmed. The debate about whether our use of fossil fuels would lead to a warming planet was over at least 50 years ago. For the last several decades, the vast majority of those that know the most about the subject have been saying that our use of fossil fuels will lead to changes in our environment that are dangerous to our way of life. Now it is past time to quit pretending we don't have a problem.
I would encourage people not to take my word for what I have said about the history of the science. All of this can be verified independently, either in textbooks or on the web. I would caution that there is an awful lot of material on the web that is complete nonsense. Think of this like the game where a message is passed from person to person and comes out somewhat mangled at the other end. Try to get your information about the research as close to an original source as you can. If you can't trace your source back to some actual research, it is as valid as a rumor started by someone who was not there.