Although I appreciate Charlie Lineweaver's "out of left field" ideas and unusual ways of viewing things, I have problems with this paper of his. It attempts to make thermal histories a very simple story when they are probably rather more complex.

I like the idea of showing the cooling of the Universe, and the way that various things become permitted by thermodynamics through time, but the hugely logarithmic timescale is rather misleading, in my opinion. Only for the first 10% or so of the history of the Universe was liquid water and free energy unavailable. For the rest of it, it has been "business as usual" or at least, something close to it.

I'm particularly disturbed by his pithy summing up. To Paraphrase "A cup of coffee cools down, the Earth cooled down. Get over it!"

This grossly conflates different ideas about thermal equilibrium. A cup of coffee cools down on a timescale determined by its radiative flux, liquid convection, thermal conductivity and thermal inertia. The Earth is VERY different. The timescale of conductive/radiative heat loss from the near-surface is essentially instantaneous compared to the geological timescale of thermal convection. In a cup of coffee, both processes have timescales of the order of seconds.

The Earth's *interior* cools down on a timescale determined by thermal conductivity, solid state convection, thermal inertia, but most importantly, Radiogenic heat generation. In a steady-state, the Earth remains warm, but a cup of coffee does not. Although many of the processes are similar, the timescales and implications are very different.

The *surface*of the Earth is in a very different thermal state - this is determined on a much shorter timescale by greenhouse gas and albedo moderation of solar thermal input and IR re-radiation.

To draw an analogy between a cooling cup of coffee and the apparent steady cooling of the surface of the Earth (via the proxy of the oceans) is very misleading. For the Earth, numerous processes must actively maintain a balance *which steadily changes with time* over BILLIONS of years. For the coffee cup, the rules are set and determine the result on a timescale of minutes.

It is an empty argument to make the comparison, and evades all discussion of WHAT processes made the early Earth warmer, whether they apply to other real and imagined planets, and how well we actually understand them.

In the light of the Global Warming debate, it is a dangerous and potential politicisable statement, that the Earth has historically cooled, and that therefore one might assume it would continue to do so in the future (unless something stops it).

This argument totally ignores the very real observation of major climate oscillatory and irregular change over geologic time and over the timescale of human civilisation.

Maybe I'm getting het-up about something small, but to me it matters. This is an area of science I've worked actively in, and it hurts me to see such unhelpful ideas presented in a professional paper.