Publikationer
I urval från Stockholms universitets publikationsdatabas
Artikel Climate policy2015. John Hassler, Per Krusell, Jonas Nycander. Economic Policy 31 (87), 501+
This paper makes suggestions for climate policy and defends them based on recent research in economics and the natural sciences. In summary: (i) the optimal carbon tax is rather modest; (ii) the key climate threat is coal; (iii) a carbon tax is to be preferred over a quantitybased system; (iv) the optimal tax on carbon does not appreciably harm growth; (v) subsidies to green technology are beneficial for the climate only to the extent that they make green technology outcompete coal; and (vi) a carbon tax is politically feasible.

2015. Jonas Nycander, Magnus Hieronymus, Fabien Roquet. Geophysical Research Letters 42 (18), 77147721
The role of nonlinearities of the equation of state (EOS) of seawater for the distribution of water masses in the global ocean is examined through simulations with an ocean general circulation model with various manipulated versions of the EOS. A simulation with a strongly simplified EOS, which contains only two nonlinear terms, still produces a realistic water mass distribution, demonstrating that these two nonlinearities are indeed the essential ones. Further simulations show that each of these two nonlinear terms affects a specific aspect of the water mass distribution: the cabbeling term is crucial for the formation of Antarctic Intermediate Water and the thermobaric term for the layering of North Atlantic Deep Water and Antarctic Bottom Water.

2013. Jonas Claesson, Jonas Nycander. Ecological Modelling 256, 2330
The most severe impact of climate change on vegetation growth and agriculture is likely to occur under waterlimited conditions. Under such conditions the plants optimize the inward flux of CO2 and the outward flux of water vapor (the transpiration) by regulating the size of the stomatal openings. Higher temperature increases water loss through transpiration, forcing the plants to diminish the stomatal openings, which decreases photosynthesis. This is counteracted by higher CO2 concentration, which allows plants to maintain the inward flux of CO2 through the smaller openings. These two counteracting effects, combined with the change in precipitation, determine the net change of biological productivity. Here, a vegetation sensitivity approximation (VSA) is introduced, in order to understand and estimate the combined effect of changed temperature, CO2 and precipitation to first order. The VSA is based on the physical laws of gas flux through the stomatal openings, and is only valid under waterlimited conditions. It assumes that the temperature depends logarithmically on the CO2 concentration with a given climate sensitivity. Precipitation is included by assuming that it is proportional to the transpiration. This is reasonable underwaterlimited conditions, when transpiration is often a large fraction of the precipitation. The VSA is compared to simulations with the dynamic vegetation model LPJ. The agreement is reasonable, and the deviations can be understood by comparison with Koppen's definition of arid climate: in an arid climate growth increases more according to LPJ than according to the VSA, and in nonarid conditions the reverse is true. Both the VSA and the LPJ simulations generally show increased growth with increasing CO2 levels and the resulting temperature increase, assuming precipitation to be unchanged. Thus, in this case the negative temperature effect is more than compensated by the positive effect of CO2.

2012. Kristofer Döös (et al.). Journal of Physical Oceanography 42 (9), 14451460
A new global streamfunction is presented and denoted the thermohaline streamfunction. This is defined as the volume transport in terms of temperature and salinity (hence no spatial variables). The streamfunction is used to analyze and quantify the entire World Ocean conversion rate between cold/warm and fresh/saline waters. It captures two main cells of the global thermohaline circulation, one corresponding to the conveyor belt and one corresponding to the shallow tropical circulation. The definition of a thermohaline streamfunction also enables a new method of estimating the turnover time as well as the heat and freshwater transports of the conveyor belt. The overturning time of the conveyor belt is estimated to be between 1000 and 2000 yr, depending on the choice of stream layer. The heat and freshwater transports of these two large thermohaline cells have been calculated by integrating the thermohaline streamfunction over the salinity or temperature, yielding a maximum heat transport of the conveyor belt of 1.2 PW over the 34.2PSU salinity surface and a freshwater transport of 0.8 Sv (1 Sv = 10(6) m(3) s(1)) over the 9 degrees C isotherm. This is a measure of the net interocean exchange of heat between the Atlantic and IndoPacific due to the thermohaline circulation.

2011. Jonas Nycander. Journal of Physical Oceanography 41 (1), 2841
A local neutral plane is defined so that a water parcel that is displaced adiabatically a small distance along the plane continues to have the same density as the surrounding water. Since such a displacement does not change the density field or the gravitational potential energy, it is generally assumed that it does not produce a restoring buoyancy force. However, it is here shown that because of the nonlinear character of the equation of state (in particular the thermobaric effect) such a neutral displacement is accompanied by a conversion between internal energy E and gravitational potential energy U, and an equal conversion between U and kinetic energy K. While there is thus no net change of U. K does change. This implies that a force is in fact required for the displacement. It is further shown that displacements that are orthogonal to a vector P do not induce conversion between U and K, and therefore do not require a force. Analogously to neutral surfaces, which are defined to be approximately orthogonal to the dianeutral vector N. one may define P surfaces to be approximately orthogonal to P. These P surfaces are intermediate between neutral surfaces and surfaces of constant sigma(0) (potential density reference to the surface). If the equation of state is linear, there exists a wellknown expression for the mixing energy in terms of the diapycnal flow. This expression is here generalized for a general nonlinear equation of state. The generalized expression involves the velocity component along P. Since P is not orthogonal to neutral surfaces, this means that stationary flow along neutral surfaces in general requires mixing energy.

2007. Jonas Nycander (et al.). Journal of Physical Oceanography 37, 20382052
Calculating a stream function as function of depth and density is proposed as a new way of analysing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this stream function directly shows whether it is driven mechanically by largescale wind stress, or ''thermally'' by heat conduction and small scale mixing. It is also shown that the integral of this stream function gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although formally there is no thermodynamic work in an incompressible fluid. The proposed method is applied both to an idealized coarseresolution threedimensional numerical ocean model, and to the realistic highresolution OCCAM model. It is shown that the overturning circulation in OCCAM between 200 m and 1000 m depth is dominated by a thermally indirect cell of 24 Sv, forced by Ekman pumping. In the densenst and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized smallscale mixing.
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