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Clearing the air

Richard Black | 14:10 UK time, Thursday, 29 January 2009

Whales, schistosomes, wind power and Antarctica all generated interesting points on recent posts and I'll try to deal with some of them. Apologies for being dilatory in responding to some of the questions raised.

Iceberg with Sea Shepherd vessel behindTo the white continent first, and the many comments relating to my article on last week's Nature paper indicating that Antarctica has warmed, overall, over the last halfcentury. (I've had lots of emails on the subject as well, so this is a good forum in which to frame a reply).

toughNeilHyde, you asked if I "could explain how the scientists have reached their findings based on 50 years of satellite records, when the sats have only been up there 30 years".

I'll give it a go. Clearly it's impossible physically to get data from satellites that didn't exist, but there is data from weather stations going back further than that.

So, if you can correlate the satellite and weather station record over the period for which both existed - how they vary together and how closely tied they are - and if you assume that the relationship between the two was roughly the same in previous decades, then you can compute what the satellites probably would have shown across the continent if they'd been operating, with appropriate error bars.

Whether you find this convincing or not is up to you. Clearly some observers disliked it, although others mounted a vigorous attack on these more sceptical observers.

I don't find this surprising at all. Even in uncontroversial areas of science, a single study can often generate widely varying reactions - and this is anything but uncontroversial.

The important thing, it seems to me, is to keep the nature of science in perspective. Single studies rarely "prove" anything; and ultimately only posterity, rather than peer review or polemic, can judge whether researchers were right or wrong.

So when you write, PAWB46, that I and lots of other journalists decided to report on this because we "fell for the media hype", to me that's missing the point.

We - or at least I - report such studies because they add to the pile of stuff that we already have on a very important issue. It's up to you to decide whether the pile consists of something marvellous or malodorous.

Wind turbines, one brokenYou also suggest, PAWB46, that somehow the Antarctica paper was kept "top secret". There's no secret - you just have to subscribe to Nature, it's a commercial publication.

One other climate-related comment to clarify: manysummits, you say that melting of the Greenland ice cap would add about 20 metres to sea levels - in fact it's more like 20 feet (six to seven metres).

Now, in the thread on renewable energy I did make an error, peroxisome, as you suggested - but it was linguistic rather than mathematical.

Referring to a study from the UK Energy Research Centre (UKERC), I wrote: "to keep the supply as reliable as it is now (and no electrical system is 100% reliable) the backup would need to have about one-fifth the capacity of the wind turbine fleet - ie if wind supplied 20% of the electricity, the backup would have to be able to supply about 4%".

What I should have written was "the backup would need to have about one-fifth the output of the wind turbine fleet" - because, as you correctly point out, the turbines don't spin all the time.

Basically, UKERC has taken the load factor into account already - it was my conflation of "capacity" and "output" that muddied the waters. Rob Gross of UKERC has sent me a more detailed explanation which I'm pasting in at the end of this entry.

A couple of quick ones on whaling. MadTom1999, you say that "blue whales and other endangered species turn up on Japanese plates".

To my knowledge, there have been no recent scientifically validated reports of blue whale meat on sale (although Japan and some other countries do take species listed as endangered, such as fin and sei whales). The Australian government acknowledged as much [Word file] last year. If you have new evidence, could you post details?

Dead blue whale on beachRustigjongens, I wrote about the issue of how upbringing and culture might inform food choices (frogs, dogs, whales) a couple of years ago while in Japan, and I'd be interested to see what you make of it.

Finally, the post on whether conserving biodiversity could be a useful way of curbing the impact of disease brought some really interesting comments, and particular thanks to DavidCrosweller for insights into the problems involved in tackling issues such as schistosomiasis and sanitation in the field.

I completely agree, ClaphamBusman, that conserving biodiversity is never going to be the whole solution. After all, the parasites have presumably evolved in a diverse ecology, so simply restoring or preserving that ecology isn't going to wipe them out.

But it seems to me that you need a diverse toolbox for this kind of issue, and if looking after your land, preventing run-off and keeping the snail population together is going to reduce the disease burden, then why not? - especially as it's likely to have other benefits in the long run.

With that, a belated Happy Lunar New Year - I'm sure the climate-sceptical among you will be able to make some play on the fact that we're now in the Year of the Bull - and here's Rob Gross's explanation of the maths of renewables back-up.

"Reliability is a capacity issue. The capacity credit of wind is about 20%, which means that it can displace about 20% of its own installed capacity in terms of the conventional capacity that can be retired (NB: at high penetrations the capacity credit for wind might fall as low as 10%). The low load factor of wind is already reflected in this (low) capacity credit. It is also reflected in the 'comparator plant' that it makes sense to compare a wind farm to.

"100MW of wind would replace the energy from a 30MW conventional plant (roughly). In order to keep peak supplies secure it could only displace a conventional capacity of 20 MW. To the extent that 'back up' is needed you need 30-20 = 10MW. 'Back up' on this basis represents the additional thermal capacity needed on the system when 100MW wind replaces the energy output of 30MW of thermal.

"To use your example: If wind supplies 20% of electricity (say 80TWh) at a load factor of 30% you require around 25GW of wind installed. The capacity credit of wind is 20%. So you can retire 5GW of thermal plant without reducing reliability. However instead of 25GW of wind you could have got the same energy (80 TWh) from 8GW of fossil stations (very roughly). Hence the 'back up' needed in this scenario is 8 - 5 = 3GW. 25GW Wind contributes energy equivalent to 8GW of thermal power. But it only contributes reliability equivalent to 5GW.

"Suppose capacity credit is only 10%. The energy contribution of wind is unchanged. The energy equivalent comparator is also unchanged. In this instance you can only retire 2.5GW. So for 25GW wind you needed 5.5GW back up. If capacity credit is zero you need 8GW. In all cases you need a larger system than you would if you had all thermal generation. But in no case do you need to cover anything like two-thirds of the wind capacity.

"Your reader gets it wrong because he suggests that we need to treat wind power capacity as if it were a conventional generator. Equivalent here to 25GW - 5GW, or 80% back up of the wind capacity.

"Instead we must consider how much conventional capacity we can retire when wind comes on the system without making the system less reliable. This will be lower than the amount of capacity we could retire if we only need to keep average energy output the same. The difference gives a crude estimation of 'back up' (though this is a misleading term). Of course all these numbers are smaller than the nameplate installed capacity of wind power required to provide a given amount of energy."


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