Science is based on evidence and arguments that are always open to the discovery of further evidence. Justification exists along a continueem. There are a vast number of issues that are still a matter of discussion and debate in the scientific community in all active areas of scientific investigation and inquiry.
But some things are well established -- like the fact that the Earth is round or that light is composed of photons. As different lines of investigation tend to support certain conclusions they become so well supported that they are no longer considered a matter for serious debate. Science progresses.
A scientific consensus exists that global warming is taking place and that our emission of carbon dioxide is playing a central role in this process. I would also argue numerous physical principles and observations are part of an interconnected network and together form the foundation for and are part of the consensus.
The foundation is strong. To noticeably weaken it would require a great deal more than for some small doubt or even serious doubt to be raised with respect to any one element or another. Serious doubts would have to be raised with respect to numerous well-established elements and result in their removal from this foundation.
Among the elements that are part of the foundation for our understanding of anthropogenic global warming I would include the following:
- The basic principles of quantum mechanics, (e.g., Schroedinger's Equation, linear superposition, the probability density operator and mixed states, quantized energy levels, spontaneous decay as an exponential function of time) that underly a large variety of technologies including lasers, tunnel diodes, photovoltaic cells, nuclear magnetic imaging used at hospitals, forms of infrared vision used in fighter jets, the satellite imaging of storms -- as well as our theoretical understanding of chemistry;
- Our understanding of the absorption of radiation by molecules by their entering quantized states of vibrational, rotational, or rovibrational excitation due to the electric dipoles of molecules and the electromagnetic nature of radiation -- essentially by means of the same mechanism that warms food or boils water in a microwave oven;
- How local thermodynamic equilibrium conditions are guaranteed when the rate at which a molecule suffers collisions is more than a million over the half-life of the relevant states of molecular excitation -- such that the spectra of the thermal radiation emitted by solid, liquid or gas is determined by the intrinsic properties of the substance and its temperature, not the radiation being absorbed;
- Radiant energy that is absorbed by greenhouse gases will be thermalized by molecular collisions under local thermodynamic equilibrium conditions since molecules undergo a million collisions or more during the half-life of a given quantized state of molecular excitation;
- Thermalization under local thermodynamic conditions will imply the sharing of energy between different states of excitation according to the equipartition theorem;
- Under local thermodynamic conditions the thermalization of energy will imply the that molecular kinetic energy obeys the Maxwell-Boltzmann distribution;
- Quantum statistical mechanics predicts/explains the specific, detailed absorption spectra of different greenhouse gases;
- Under conditions of local thermodynamic equilibrium, absorptivity and emissivity are equal in accordance with Kirchoff's law as a consequence of the second law of thermodynamics, where Kirchoff's law has been well-established since the late 1800s;
- Carbon dioxide is a greenhouse gas - absorbing infrared radiation - well-established since the late 1800s;
- Given the shallow scale height of water vapor and well-mixed distribution of carbon dioxide, the narrowing of spectral peaks with decreased atmospheric pressure, the difference between spectral lines and spectral bands (such that even if spectral bands overlap spectral lines between different greenhouse gases may not), the atmosphere is in fact not saturated in the relevant parts of the absorption spectra of carbon dioxide and therefore increasing the levels of carbon dioxide will result in greater absorption of infrared radiation;
- Atmospheric carbon dioxide has been increasing since at least the 1950s (instrumental readings);
- The rate at which carbon dioxide has been increasing and its isotopic signature strongly implies that the origin of the additional carbon dioxide lies in the combustion of fossil fuel;
- Concentrations of oxygen have been decreasing over time in parallel with the rise in the levels of carbon dioxide likewise suggesting that the origin of increased levels of carbon dioxide lie in the combustion of fossil fuel;
- Satellite imaging of increased levels of carbon dioxide demonstrates how carbon dioxide reduces the rate at which thermal radiation escapes the atmosphere;
- In accordance with radiation balance theory which is grounded in the principle of the conservation of energy, given the reduction in radiation that is escaping the atmosphere and the principle of the conservation of energy the amount of thermal energy in the climate system must increase;
- Since the thermal radiation emitted by a warm body is proportional to the fourth power of the absolute temperature, the temperature of the effective radiating layer (where radiation tends to escape without re-absorption -- but which rises to colder altitudes given rising concentrations of atmospheric carbon dioxide) must warm if the rate at which energy leaves the climate system is to once again balance the rate at which energy enters the system;
- When the higher altitude warms up, given the lapse rate (the rate at which temperature drops with altitude in the troposphere) and the increased distance between the effective radiating layer, the surface must warm;
- Given a rise in temperature absolute humidity will rise in accordance with the Clausius-Clapeyron relationship where this relationship has been well-established since the late 1800s;
- Rate of evaporation and water vapor partial pressure increase by roughly 8% with every degree Celsius and double with every 10 degrees Celsius;
- Since water vapor is a greenhouse gas, this will act as positive feedback, further raising the temperature;
- Given instrumental readings at by surface stations, at sea, at different ocean depths, by means of satellite, borehole readings, the accelerating decline of glaciers the accelerating decline of arctic sea ice during its annual minima, the migration and changing of the annual cycles of plants and animals, the measurable expansion of the Hadley cells of atmospheric circulation and consequently the subtropics, the rise of the tropopause, etc we have many different lines of evidence any one of which strongly implies global warming -- which pretty much syncs up with our understanding of the physics;
- But for the solar cycle, solar output has remained level to slightly declining over the latter part of the 20th Century (roughly since 1960) even as the modern age of global warming took off in 1975;
- Greater solar radiation would warm the upper stratosphere as well as the troposphere whereas an enhanced greenhouse effect would cool the upper stratosphere as the troposphere warmed -- and the upper stratosphere has cooled as the troposphere has warmed; and,
- Greater solar radiation would warm the days more rapidly than nights whereas an enhanced greenhouse effect would warm the nights more rapidly than days -- and the nights have warmed more rapidly than days.