Manuscript prepared for J. Hobbitlore with version 5.0 of the LATEX class copernicus.cls. Date: 3 December 2013 The Climate of Middle Earth Radagast the Brown1,2 1Rhosgobel, nr. Carrock, Mirkwood, Middle Earth. 2The Cabot Institute, University of Bristol, UK. Correspondence to: [email protected] Abstract. In this paper, I present and discuss results from predictions at a lower spatial resolution (typical scale of hun- a climate model simulation of the ‘Middle Earth’ of elves, dreds of kilometers as opposed to kilometers). Climate model dwarves, and hobbits (and not forgetting wizards such as my- predictions are an integral part of political and societal plan- self). These are put into context by also presenting simula- ning for the coming decades to centuries, and the recent re- tions of the climate of the ‘Modern Earth’ of humans, and port from the Intergovernmental Panel on Climate Change of the ‘Dinosaur Earth’, when dinosaurs ruled the Earth 65 (IPCC) summarises many of these future predictions (IPCC, million years ago. 2013a). Several aspects of the Middle Earth simulation are dis- Because climate models are based on fundamental scien- cussed, including the importance of prevailing wind drec- tific understanding, they can be applied to many situations. tion for elvish sailing boats, the effect of heat and drought They are not designed solely for simulating the climate of on the vegetation of Mordor, and the rain-shadow effects of the modern Earth, and, in theory, the same underlying science the Misty Mountains. I also identify those places in the Mod- should apply to any time period in the past. The only caveat ern Earth which have the most similar climate to the regions is that in order to simulate climates different from modern, of The Shire and Mordor. the user must provide some ‘boundary conditions’ - maps or The importance of assessing ‘climate sensitivity’ (the re- variables which are not predicted by the model. Examples sponse of the Earth to a doubling of atmospheric carbon diox- include spatial maps of the height of the global terrain (to- ide concentrations) is discussed, including the utility of mod- pography) and ocean depth (bathymetry), characteristics of elling and reconstructing past climate change over timescales rocks and soils, and concentrations of key atmospheric con- of millions of years. I also discuss the role of the Intergov- stituents, such as ozone and carbon dioxide (CO2). In addi- ernmental/Interkingdom Panel on Climate Change (IPCC) in tion, key parameters such as the strength of the sun, and the assessing climate change, and the responsibilities placed on radius and the rotation rate of the planet, also need to be pro- policymakers. vided to the model. Adapting the model to simulate past time periods is po- tentially very powerful because, in theory, we can know the ‘right’ answer from observations, and test the performance 1 Introduction of the models by comparing their results with these observa- tions. For time periods prior to humans making careful ob- Computer models of the atmosphere, land surface, and ocean servations of the weather, we rely on indirect observations are routinely used to provide forecasts of the weather and cli- of many aspects of past climates, such as information from mate of the Earth. They are based on our best theoretical un- tree rings and ice cores, and fossils of plants and animals. derstanding of fluid motion, physics, chemistry, and biology, However, provided that we understand the uncertainties and written in the form of equations, and then converted into a errors in these ‘proxy’ records of past climate change, and form which can be solved by a computer. provided we also understand and account for uncertainties in Climate models and models used to make weather fore- the boundary conditions we apply to the model, we can make casts are very similar to each other, except that climate mod- use of past periods going back millions of years, to time pe- els typically simulate longer periods of time than weather riods when the Earth looked very different from the modern. models (years to centuries as opposed to days to weeks), and therefore, due to limits on computer time and power, make 2 R. Brown: Climate of Middle Earth In addition, by varying the topography/bathymetry, the ro- spheric turbulence, or eddies in the ocean. It is the represen- tation rate and radius of the planet, and density of the atmo- tation of these sub-gridscale processes which brings uncer- sphere, we can, in theory, use climate models to simulate any tainty into climate modelling (the equations of fluid motion planet, real or imagined. and thermodynamics themselves have been known and un- In this paper I present three climate model simulations, of derstood for several centuries). As well as the atmosphere the Modern (pre-industrialised) Earth, of the Dinosaur Earth and ocean, the model includes a representation of the land (a time period called the Late Cretaceous, about 65 million and ocean surface, including processes associated with sea- years ago, just prior to the extinction event which killed off ice, soil moisture, and, in our particular version of the model, the dinosaurs), and Middle Earth - the land of hobbits, elves, the growth and distribution of vegetation. dwarves, wizards, and orcs. The model is given an initial state of all the variables The aims of this paper are threefold: which it predicts (for example temperature, pressure, wind (1) To demonstrate the flexibility of climate models, aris- speed, snow cover, ocean density), and then the model is ing from their basis in fundamental science. ‘run’ forward in time, in steps of typically 10 to 30 minutes. (2) To present the modelled climate of Middle Earth, and Weather systems develop and evolve, rain falls, the seasons provide some lighthearted discussion and interpretations. come and go, and years of ‘model-time’ pass (for my model, (3) To discuss the strengths and limitations of climate one year of model-time typically takes about 2 hours of ‘real- models in general, by discussing ways in which the Middle time’). Finally, the weather predicted by the model in the fi- Earth simulations could be improved. nal years or decades of the simulation are averaged, resulting in a model-predicted ‘climatology’ - the climate, or average weather, predicted by the model. 2 Model description HadCM3L is a relatively complex model, known as a ‘General Circulation Model’, or GCM. However, it is not a I use a climate model developed at the UK Met Office, state-of-the-art model, and includes less processes and has ‘HadCM3L’, which is capable of simulating the atmosphere, fewer gridboxes (i.e. runs at lower resolution) than more re- ocean, and land surface. In common with most climate mod- cent models, such as those used in the most recent IPCC re- els, HadCM3L represents the world in ‘gridbox’ form, with port (IPCC, 2013a). However, it is useful for my purposes, a 3-dimensional network of boxes covering the surface and as its relative efficiency of computation means that it can be layered to extend up to the top of the atmosphere and down run for a sufficiently long time to reach an equilibrium, given to the bottom of the ocean depths. The size of eaach box is that the initial state I put the model into may be quite differ- 3.75 degrees of longitude by 2.5 degrees of latitude, with a ent from the final predicted climate. height dependent on the distance from the Earth’s surface - boxes situated near the surface of the Earth have a smaller height than those at the top of the atmosphere or bottom of 3 Experimental design the ocean. This results in a ‘matrix’ of boxes covering the world, with 96 boxes in the West-East direction, 73 boxes in In this paper I present results from 3 climate model simula- North-South direction, 20 boxes deep in the ocean, and 19 tions using HadCM3L. The first is a simulation of the Earth high in the atmosphere (a total of more than a quarter of a during the period prior to large-scale industrialisation (for million boxes, although not all are used as some are effec- sake of argument, the period 1800-1850). I call this sim- tively below the sea floor). ulation ‘Modern Earth’. The second simulation, ‘Dinosaur In this matrix, the fundamental equations of fluid motion Earth’, is of the period just prior to the extinction of the di- in the atmosphere and ocean are formulated and solved, with nosaurs (the Late Cretaceous, ∼65 million years ago), and the additional complication that the Earth is spinning on its the third, ‘Middle Earth’, is of the climate of Middle Earth. axis. Energy is added to the system due to absorption of light The model setups for these three simulations are very sim- and heat radiation emitted by the Sun, and energy leaves the ilar. The only important differences are the boundary condi- system through emission of heat or reflection of light radia- tions. tion into space. All variables in the model can be considered For Modern Earth, I use the standard pre-industrial global as average values over the volume of each gridbox, and so boundary conditions, provided by the UK Met Office, which the climate model can not provide any information at a spa- are derived from observations of the modern continental con- tial scale smaller than one gridbox (so, for example, although figuration of the Earth, topography, bathymetry, and land- it makes sense to talk about the modelled climate of the UK, surface characteristics. This simulation uses a CO2 concen- or Mordor, the model can not give information about Bristol, tration of 0.28% (280 parts per million, or ‘280 ppm’) of or Bree).