It doesn't take much to spark the imagination. Looking at a map of the Earth, it's easy to see how easily the Americas could snuggle up to the Old World should they be so inclined. This was first notice back in the 1600s, but no one thought much of it beyond "Huh." It was given more weight in the early 20th century when "continental drift" was elucidated by Alfred Wegener. Even then, it was very controversial because there was simply no known mechanism for how the continents could move; it was derided that he had suggested that the continents simply plowed through oceanic crust. However, evidence and hypotheses accumulated through the 20s, 30s, and 40s and continental drift developed an interesting fringe notion into an established, albeit still controversial, scientific theory by the 1950s.
By the mid-60s, gravimetric, seismic, and paleomagnetic1 evidence had accumulated to the point that the old understanding of the Earth's crust was thrown out and plate tectonics was an established theory, and those denying it were increasingly relegated to the fringes. Today, only those completely ignorant of the extensive evidence argue other hypotheses to explain the grossest facts. We have a consensus.
Science isn't a hierarchical organization; there's no top-down, centralized authority that decides what scientific truth is. Each scientist examines the evidence and draws conclusions. Each scientist gathers evidence and publishes conclusions, hoping to convince others. The more compelling their argument, the more convincing they are, the more they contribute to global understanding. Individual scientists can carry great weight in their field. Truly eminent figures such as Pauli, Dirac, and Feynman could be terrifying to young physicists, shredding their work and humiliating them in public. However, not even they were empowered to make pronouncements on scientific truth ex cathedra. Many older scientists fear becoming the increasingly awkward presence at conferences, no longer contributing to valuable new research, but increasingly devoted to odd ideas of no merit, wasting their twilight years in pseudoscience, mocked by their younger peers. Even scientists in their prime, recognized experts in their fields making substantial and valued contributions in areas of interesting and controversial research, when they step out of their laboratories to opine on fields not their own, meet rolling eyes and barely constrained disdain.
Scientific consensus isn't handed down from on high, nor is it decided upon by vote. It stems neither from authoritynor from popularity. Rather, consensus emerges as the evidence for or against a proposition accumulates and scientists find themselves increasingly swayed, not by other scientists, but by nature itself. Consensus is an emergent, organic process. The blind men might initially mistake the elephant for a wall, a pipe, a snake, but through cooperation and comparison they build to the truth and learn that each is perceiving an aspect of the whole.
Consensus on large, complex issues takes decades to achieve. The global phenomenon of plate tectonics had results that were apparent to cartographers of the seventeenth century as a phenomenon in need of explanation, but there was no way to study it until the twentieth century, and even then it took fifty years. Darwin's theory took the scientific world by storm because it was the culmination of decades of careful work on his part, but also because it was the culmination of centuries of observational science. Geology and biology had mountains of evidence seeking an explanatory framework, and the controversy between opposing and incorrect schools of thought were impossible to reconcile. Evolution provided a fundament on which biology could build a vast, interconnected structure that explained all of their observations and which dovetailed perfectly with the geological evidence. It was a consensus waiting to happen.
The prospect of global warming was raised in the nineteenth century. Observation of changing temperatures appeared in the twentieth. Dedicated research began in the 1950s and the science of climate change was born. Models have been developed over decades, and techniques innovated for gathering more detailed evidence across wider historical ranges. In the 90s, consensus had emerged among climate scientists; we're causing warming. As the models have gotten more sophisticated and the issue more attention, consensus has spread to the larger scientific community and evolved from "global warming" to "global climate change". Evidence has accumulated to the point where, as Stephen J Gould might put it, "it would be perverse to withhold provisional assent."2
This is all different from dogmatic assertion. It is arrived at independently by every scientist. It isn't mere speculation, absent a physical referent, but is built upon a foundation of evidence. It isn't a theological doctrine, invented whole cloth and never to be questioned, but is arrived at after decades of painstaking work responding to thousands of difficult challenges. Scientists have no dogma. They have observation, evidence, curiosity, and discipline. Each dreams of being the next Newton or Darwin, providing a powerful framework to draw together centuries of observation. Each hopes to be an Einstein or Planck, overturning an old modality and uncovering a deeper truth. Each hopes to be a controversial iconoclast, blazing new trails into uncharted truth, but settles for a life of quiet investigation.
The irony at the core of those who oppose science is that they accuse science of harboring their own greatest weakness, dogma, and attack it on the basis of its greatest strength, dispute. Evidentiary dispute lies at the heart of science, wherein competing hypotheses are tested against the facts and the intellects of opposing egos, insuring that unsupported ideas are discarded and only those explaining all the facts and contradicted by none can survive. And this is called a weakness by doctrinaire opponents of scientific investigation, as if only hierarchical agreement with a central dogma can be an approach to truth. And then, when those disputations arrive at powerful and supported truths, they attack that as dogma, when it is anything but.
1 - Gravimetric - Measuring gravity. For example, using the world's most sensitive plumb bob to figure out how much a mountain weighs. Seismic - Measuring earthquakes. Every time an earthquake happens, the earth rings like a bell. By recording when the energy from an earthquake at point A arrives at point B, geologists have built up a picture of the interior of the Earth. It's cool. Paleomagnetic - Measuring the magnetic fields in rocks. The magnetic field of a rock reflects the magnetic field of the Earth when the rock was formed. This allows you to determine the twistings and turnings and shiftings and foldings the rock has taken since it was laid down.
2 - 'In science, "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional assent." I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.' - Hen's Teeth and Horse's Toes