Batgung

The message from the Hong Kong Observatory is unequivocal: global warming is real; it’s affecting Hong Kong’s recent and current weather in obvious ways; and future trends for the world, and perhaps for Hong Kong especially, are ominous.

Climate change pops up in all corners of the HKO's website. From their extensive package of climate change resources for the general public and education, to the HKO Head's blog, there’s a consistent focus on forming public opinion and changing people’s behavior.

Frequent reminders of the implications of climate change also appear in their other publications. Take as a recent example the HKO’s year-end round-up of 2009’s weather. It begins by stating that the World Meteorological Organization expects 2009 to be among the top 10 hottest years on record. And, in line with this, the report notes that Hong Kong’s 2009 temperatures came in at 9^th place in our own list of hottest years, and that 30 very hot days were recorded in 2009, which was the most since 1963.

Next, the 2009 round-up reminds us of Hong Kong’s warming trend by providing a little table highlighting the top 11 hottest years on record here; if you have a look at it (by using the link above), only one of those 11 years precedes the 1990s, and six of them occurred in the 2000s.

So it’s getting hotter, and the HKO sees this as the most salient fact to report about 2009’s weather. But are there any other patterns to note, any other helpful angles we might find in these numbers?

Given the recent dust-ups in the climate change arena (see for example here, here  and here, just to get the smallest taste of the vast controversy that is currently roiling the entire ‘science’ of climate change) I decided to launch my own little Hong Kong-specific investigation.

Since I’m no climate scientist – and since I’m pretty lazy – I thought I’d better keep things simple at the outset. So I kicked off by conducting my own informal survey of temperature averages in HK over the past 13 years (mostly since those numbers are so easily available on the HKO website). 

I’ve made up a table that sets out the yearly deviations from normal in overall average temperatures, plus the deviations from normal average highs and lows, in the years since 1997. I’ve also highlighted how those years ranked (at the time they were reported) in the all-time records.

Do we learn anything new from this additional data?

Well, the first thing that struck me is that yearly temperatures in HK are remarkably uniform. 1998 averaged just one degree Celsius above normal*, and that's Hong Kong's all-time heat record. A mere 0.4 degrees above normal (as in 2009) merits a spot in the top ten. Many places in the world have much wider yearly variations. So clearly we’ve seen some warming, but we can’t be talking about a big number, at least not yet. But of course it’s the rate of warming that’s important, so we can’t jump to any rash conclusions.

*We do need to keep in mind that the HKO uses a rolling 30-year span to establish their ‘normal’ temperatures. That is, the current ‘normals’ are the averages from 1971-2000. Every decade, therefore, the period for the averages is moved 10 years ahead. And since Hong Kong is getting steadily warmer, the deviations from ‘normal’ in recent years will be smaller than they would be if they were compared to temperatures from the whole span of the HKO’s records. This paper outlines this trend in much more detail. For example, it notes that the 1961-1990 daily average high/low was 25.7/20.9, whereas from 1971-2000 these numbers changed to 25.6/21.1.

Second, you can see that, according to the HKO, all recent years have recorded above-average yearly temperatures – save for 2008, which was right on normal – and that 11 of the past 13 years have been top-10 on record for heat at the time. This is certainly in line with the HKO’s warnings about climate change.

But now let’s look a bit more closely at the average highs/lows.

First, take a peek back at 1997, 2000 and 2004. According to the HKO, these were all among the top ten hottest years in Hong Kong history (and the records go all the way back to 1884). But those three years all recorded average high temperatures that were actually below normal. How can that be?

If you look at the ‘Nighttime lows’ column, you can see that almost all of Hong Kong’s recent heating has been at night. Every year – again, except for 2008 – the average nighttime low has been at least 0.5 degrees above normal, and over the whole period our nighttime lows have averaged close to three-quarters of a degree above normal, which is noteworthy. Hong Kong’s nights are clearly warmer than they used to be.

The daytime highs tell a somewhat different story. In four of the past 13 years the daytime highs have averaged below normal, and over the whole period, our daytime highs have averaged less than two-tenths of a degree above normal. This is a much smaller number, especially since a record El Nino event in the late 1990s pumped up temperatures world-wide.

Never the less, at least at first glance, it looks like Hong Kong is a poster child for climate change, and the HKO is right on in sounding the alarm.

Now, just to prove that I’m not that lazy, and to add a bit of depth to these initial observations, I went and had a look for some more detailed resources on the HKO website.

And I found plenty. In fact, the HKO provides a very accessible and comprehensive set of material on climate change and its implications for Hong Kong. There’s so much that I’ll just get started on working through it in this article.

Let’s first take a look at an HKO paper from 2004 that’s very helpful indeed: HKO Technical Note 107, ‘Climate Change in Hong Kong’.

This document traces out historical trends in Hong Kong’s weather, with a focus, naturally enough, on temperatures. The HKO’s records go back to the 1880s, so that gives us a pretty good longitudinal sample. Let’s see, in graphical terms, what’s been happening to Hong Kong’s average temperature in the past 125 years:

Yes, just as we might expect, there’s a steady upward trend, with an ominous spike (the red line on the graph) in recent years. The graph ends at 2002, but as we’ve already seen, nothing in the past few years is going to alter these lines by much. But since we noted above that there are big differences in recent years between daytime highs and nighttime lows, let’s see how those numbers look over a longer time frame:

These numbers are only provided post-WWII, but they’re certainly striking in the way they uphold the trends we noted earlier: Hong Kong’s nighttime lows have been increasing sharply, while its daytime highs are flat (they’ve actually declined just the tiniest bit over the past 60 years, although not to a statistically significant degree).

We’ll discuss what reasons might lie behind this discrepancy in the next installment, but for now let’s round things off by looking at temperature trends from one other angle. It’s a tenet of climate change proponents that temperature extremes are on the increase, along with rising average temperatures. Has this been the case in Hong Kong? One way to assess this is by having a look at the HKO’s data on the number of very cold and very hot days each year. They’re set out the next graph, which again covers the years from 1947-2002.

So it’s again a case of ‘yes and no’. Yes, the number of cold days, i.e. with lows of 12 degrees or below, are far less frequent, dropping from an average of about 30 in 1947 to 13 or so in our current decade. But perhaps surprisingly, no, the frequency of very hot days (33 or above) has not risen at all – in fact, it has also dropped, although far less precipitously, from about 13 to 10.

But those numbers are a bit old, and the weather in last few years may be sticking in your mind. It sure was hot late last summer, wasn’t it? And wasn’t there a cold spell in early 2008? Are the ‘extremes’ in Hong Kong getting more extreme this decade – especially the heat?

To answer this final question, I did another little seat-of-the-pants survey of the daily highs and lows over the past few years in order to extend the data in the graph above. (And may I emphasize the extremely casual nature of this survey; I will not guarantee the numbers below, but they should at least be pretty close.

The number of very hot days in Hong Kong – fueled by a truly infernal streak of sweltering days in late August/early September 2009 – has indeed increased a bit, i.e. averaging 14.8.

On the other hand, there has also been a noticeable uptick in the number of cold nights. If you look back at the HKO’s graph, you will see that the last period in which we averaged over 20 cold nights a year was the mid-70s. And it was back in the mid-80s when we last had over 30 cold nights, as 2008 did.

So the last few years were indeed a bit more ‘extreme’ than usual, at least in terms of cold nights and very hot days. But although this may fuel a bit of speculation, remember that a sample size of just six years is not really very significant.

On the whole, therefore, this initial tour through the temps has revealed a couple of salient facts. It has been getting warmer at the HKO for the past 125 years – and this warming is happening because in general (at least until the past few years) it’s not getting as cold at night as it used to. The differences we see between Hong Kong’s daytime and nighttime temperature trends are so obvious that I think they deserve further attention. We’ll give them just that in the next article.