Surface albedo enhancement

As I'm concious I'm running out of time to complete this, I'm going to lump several surface albedo techniques into one; and evaluate which one I think is better.  Rather than dwell on scientific papers I'll link to some I've read on the subject incase you want to challenge my opinion!

The idea of enhancing surface albedo
It's less efficient that enhancing atmospheric albedo, as less radiation reaches the surface, but it is obviously much more practical.  Albedo is a measure of how much incoming radiation is reflected back to space, an is measured on a scale of 0-1, where 0 is 0% reflection and 1 is 100%.  In context, the albedo of black asphalt is about 0.1, whereas fresh snow is about 0.9 (Gaskill).
Total reflectivity of the Earth's surface is measured at around 0.09, which is an averaged value.  Different landsurfaces have different albedo values, which should be accounted for in more localised studies of albedo effects.  The book by Alvia Gaskill has some good summary chapters; it's short and readable and available online.

Ways of enhancing
This idea isn't new; it's commonly known that white keeps you cooler and black makes you hotter in the summer.  I'm looking at 3 larger-scale ideas to keep the whole planet a bit cooler.


Cool Roofs

'Cool roofs' reflect more sunlight and absorb less radiation than your average dark roof.  This is achieved in 3 ways:

• Being inherently cool - i.e. the roof is made from a material with high reflectivity, for example thermoplastic white vinyl.
• Coated roofs - new or existing roofs are made reflective my applying a special coating.  You can look at coated roof product ratings here to decide on the best one.
• Green roofs - A plant is grown on the roof - it reflects less and absorbs more, but this absorbed heat is balanced by evapotranspiration.

Energy saving from this technique is partly dependent on climate.  If used in areas with cold winters, the benefits felt in summer may be cancelled out in winter with the need for heating energy (Levinson and Akbari 2010).  However, in consistently hot climates and areas where the urban heat island effects are felt; this technique is a good idea.  Urban heat islands increase energy demand (for air conditioning), add air pollution, increase heat-related illness and other detrimental effects. 
There are loads of case studies with regards to cool roofs but I have chosen a recent one from India just to give some exemplary figures:
-Buildings with cool roof coatings were measured for energy savings and emissions reductions.
-Higher reflectance lowered roof temperatures and heat flux throughout the buildings in comparison to roofs with low reflectance.
-As a result, cool roofs saved 20 to 22 kWh/m² roof area of energy, and used 14-26% less air conditioning energy.
-Direct CO2 reductions due to lower energy usage were estimated to be 11–12 kg CO₂/m² of flat roof area per year (Xu et al. 2012).
Indirect benefits from white roofs can be felt in the environment around the buildings, such as cooling outside air, reducing GHG emissions and decreasing smog, leading to better environmental and health status.  Doing this globally can reduce energy and its associated GHG emissions.  70% of buildings in the USA can improve the reflectivity of their roofs (Jo et al. 2010), and the US government take this technique very seriously as they will install a cool roof on any new building or replacement of an old roof for free.

I think there are a huge potential for cool roofs and it's something at a local scale which can be easily applied to help on a global scale to cool the planet.


Desert reflectors

In my opinion this idea seems a very farfetched - covering 60,000km2 of the desert in a reflective sheet? Although potentially it will lead to net cooling very quickly (ScienceDaily 2008) and offset the current level of warming, it does nothing to ease our reliance on fossil fuels or removal of carbon.  Also the above article says it is a fairly minimal cost of $280 billion, but that's one of the most expensive techniques I've come across so far, perhaps even the most costly (if you look back to my introduction)! An unpublished paper by Irvine et al (in press) suggests this technique should not be favoured over atmosphere or space based SRM methods.  It states these desert mirrors will alter circulation and change the hydrological cycle on a global scale; by lessening precipitation over land and reducing the Indian and African monsoons by as much as 50%.  Maybe utilising solar power in desert regions may be a more effective use of land.


Reflective Crops

Potentially a 'high albedo diet' could also contribute to global cooling - this involves normal crops being replaced with GM'd ones, with a strain of higher reflectivity.  The technique could take as long as 10-15 years to become effective, as once farmers are encouraged (probably with money) to adopt the new GM varieties, they also need time to be swapped and grown.  If anything farmers are likely to see an increase in productivity as a result of using reflective plants, which is also a bonus.
Some figures then - pioneers Ridgwell et al (2009) suggest areas of the world with intense agricultural practices, such as Eurasia and much of N. America could experience summers cooler by at least 1°C.
Unfortunately in a global context, it won't be enough to to offset the positive forcing of doubling CO2 (3.7W/m²), it will only give 0.44W/m² of globally averaged negative forcing (Lenton and Vaughan 2009).  However it is important to remember in this case every little helps, and this is affordable and achievable!


Other

Just as an aside, techniques such as modifying ocean albedo have been suggested (by applying more of that expensive plastic sheeting, adding bubbles amongst other ways) but I won't dwell on them because I don;t think they're particularly important.

In Conclusion:
As with anything, these 3 ways of altering surface albedo have their associated pros and cons.  Cool roofs is good at a local scale for reducing the urban heat island effect - the effect is also measurable and the technology is readily available and comparable.  The technique has hundreds of case studies associated with it and is probably best at indirectly cooling the planet by reducing the demand for electricity.  It does nothing to remove CO2 from the atmosphere and is dependent on climate, but is backed by many reputable organisations and governments on a global scale.
I think it's too risky to implement desert reflectivity sheets, as they alter global climate in a huge way.  It has too many unsafe effects compared to the other 2 techniques - it is also extortionately expensive and when viewed within the bigger picture of global geoengineering methods; there are cheaper and more effective methods available.
Lastly, enhancing reflectivity of crops.only for arable areas. sees more local change.  I very much like this as I'm in favour of GMing things for our benefit.  Although this has lots of moral and ethical issues attached, so does any geoengineering technique and I think it's great if we can modify something we already have for our own benefit without causing any harm.  There are very few risks associated with this and it is fairly simple to achieve, as well as being cost effective.  Unfortunately, it is limited to arable areas of the world, and produces more local changes which means it is not entirely suitable for producing global cooling.
The winner then of the best method of surface albedo enhancement is 'cool roofs', due to it's global applicability and proven effectiveness, the low level of technology involved and the amount of positive case studies associated with its implementation.