Nikolai Kardashev is the deputy director of Russian Space Research Institute at the Russian Academy of Sciences. Since the 1960’s, he has been responsible for a number of major contributions to SETI and related fields, both directly to the Soviet programs related to Extraterrestrial Intelligence, as well as IAU operations on that topic. He is perhaps most well-known for the Kardashev Scale, taken from a paper he published in 1964.
The theory behind the Kardashev Scale is that as a civilization becomes more technological advanced and extends its reach further and further out into the universe, its energy requirements will grow along with it. Since all life as we know it must consume energy to interact with our universe, we can use the energy consumption of a civilization to judge its advancement.
Kardashev originally defined a Type I civilization as consuming the amount of energy humans used at the time of his paper’s publication (1964), or about 4 TW. A Type II would be a civilization that consumed energy equal to its home star’s entire output (about 400,000,000,000,000 TW). A Type III civilization would consume energy roughly equal to its own galaxy’s output. The number of terawatts would be unwieldy to write here, so I’ll just say it is one-hundred billion times the energy consumption of a Type II (or ~ 4 x 1037 watts.)
In 1973, Carl Sagan suggested that the scale’s utility could be improved by adding intermediate values (providing fractional type values, such as a Type 1.1 civilization), and the scale would be revisited by Guillermo Lemarchand when he considered detectability of alien civilizations in 1992. Lemarchand’s major contribution for our purposes was to move the rating for a Type 1 civilization up to the total solar energy impacting the Earth, which demotes modern-day human civilization to approximately a Type 0.72 civilization.
Slower Than Light uses the revised Kardashev scale as a mechanic for gauging what challenges the player is capable of handling. The game will handle up to about a Type 2.0 civilization (indeed, achieving Type 2.0 would likely indicate a player victory, even in a sandbox mode.) The gameplay director will use energy consumption as a metric to decide which of its library of crises are appropriate for the player. For example, for Earth in 2014 (Type 0.72) an incoming large asteroid impact would be a planetary-scale crisis that needs to be dealt with successfully, or the civilization will collapse. Humanity at Type 1.0 would probably be able to handle that situation without a strategic taxation of its resources.
A Type 1.0 crisis might be a solar flare of particularly aggressive demeanor approaching the Earth. Such superflares have never been observed coming out of our sun, but have been found in similar stars. They may be products of interactions with Hot Jupiters, or it may be possible for our sun to produce such a flare otherwise unprovoked. Either way, stopping such a flare or recovering from its effects might be in the realm of a Type 1.0 civilization.
As the Type gets higher, what it takes to pose a challenge to that civilization gets more impressive. At Type 1.5, a rogue planet set to alter Earth’s orbit (if not impact directly) would be a threat of epic magnitude, although by that time Earth would likely only be one of several planets humanity controls; losing it might no longer be an existential threat to the species.
Of course, certain crises carry the same weight no matter how advanced the civilization is technologically. Political issues, such as revolutions and power struggles, will simply turn the technology humanity already has back on itself. Biological or nanomechanical dangers have a minimum level of advancement required to become an issue, but can affect any civilization above that level.
Finally, there’s always the lurking possibility of an Outside Context Problem, usually in the form of a brushing encounter with another civilization much further up the scale than the base civilization. Whether ETI will be making an appearance in STL is still an unresolved question, but such an encounter could easily be manifest as any of the crises listed above, or it could be an entirely different problem altogether.