Agriculture under future climates Karl Braganza
Yeah, thanks for having me here today. As Andrew said, yesterday our State of the Climate
report came out. Every two years we put out a report on climate change, essentially, across BOM
and CSIRO. My section actually puts out climate reports every week, every month, every season,
and every year, so that's really reporting on climate variability, what has happened, describing
what's happened and some analysis as well. But every two years, we also do this State of the
Climate report, which is largely around the issue of climate change.
Moving on, why this report? Really, it's about the change in atmospheric chemistry, so while we
will report on climate variability in a descriptive context every week, every couple years we will
come together to talk about what is a quite remarkable change in atmospheric chemistry. So this
is reported on internationally by groups like the IPCC, it's very important in Australia that we
have two agencies in BOM and CSIRO. They're actually synthesising the information for the
local stakeholders and community, and that's what we do.
And this plot's basically showing why this reporting exists. So this is carbon dioxide
concentrations in the atmosphere over the last 800,000 years. And this natural variability is
carbon dioxide actually responding to orbital changes in the Earth's orbital parameters, which is
glaciation and deglaciation, what most know as ice ages. And CO2 in the atmosphere responds to
that, it doesn't drive it, it responds to it. In the last 200 years, what we've seen is mechanical
action driving CO2 up in the atmosphere.
As Peter Hannam pointed out this morning, whether you're in the business of mitigation or
adaptation in a risk management framework, we need agencies reporting on what is going on
here and what implications that has for the climate system now and going forward. So basically,
I'm going to summarise this report through the slides that I'm going through, and just about every
word that you see here is taken out of the report. It's a synthesis report, it probably errs on the
conservative side, you need agreement across about 50 scientists across both organisations on the
wording in this document, so I think it probably hits the right note in terms of reflecting the
science.
So what does that look like when we drill down and find detail? And this is out of the report, and
what we've got is increasing CO2, it's nearly 400 parts per million. You have to go back 2.5
million years to an epoch known as the Pliocene to get atmospheric CO2 concentrations within
that ball park. When the Earth was about two to three degrees warmer you had sea levels about
20 metres higher, somewhere on the order of 10 to 20 metres higher than today at equilibrium
once everything's pushed out.
So, how did we get this increase? What we've got up here is the sources of CO2 going into the
atmosphere. So, most of it is actually from fossil fuel burning, and there's a small component due
to the land use changes, which is mostly land clearing, which is actually tailed off largely due to
changes in South America and Australia as well. The atmosphere, and the biosphere, and the
oceans, and the sea ice in the cryosphere try to take as much of this out of the system as possible.
So, the CO2 emissions are a small fraction of the total variability of CO2 due to things like the
biosphere in the northern hemisphere breathing out and breathing in over the seasons. So about
30% of the additional CO2 into the atmosphere goes into the ocean, and about 30% is taken up
by growing forests, and trees, and elsewhere. That leaves about 40% that has increased on a
compounding basis in the atmosphere. And so that 40% is the thing that's changed atmospheric
chemistry.
It's a good thing to acknowledge that no one has an intuitive understanding of what a 40%
change in an atmospheric chemical constituent should do to the climate system. You can have a
very good intuitive understanding of the climate system at your local level, you can have a very
good understanding of the cyclical nature of climate at your local level, but no one has an
intuitive understanding of what changes in atmospheric chemistry should do to the global climate
system. I think that's a good starting point for the scientific basis of what we're talking about.
And this rather complicated figure over here basically shows how we know where that additional
CO2 has come from. We basically check the Carbon-13 to 14 isotope ratio, and that tells us very,
very clearly that the additional CO2 in the atmosphere has come from fossil fuel sources. And
that's led to changes in the global climate system. So, what we've done, here, is collect together
basically all the main indicators that the climate system's warming up.
In the popular media, there's often a focus on a thing called global mean temperature, global
mean temperature isn't used much in what they call, detection attribution studies, so working out
cause and effect. Typically, we look at multiple lines of evidence, and these are some of those
multiple lines of evidence going through. The troposphere, or the column of the atmosphere
between the surface and the stratosphere has warmed up, we've got increased water vapour in
that atmosphere as its holding capacity increases. We've got a net decrease in glacier volumes,
globally, a decrease-- particularly in arctic sea ice extent-- increased air temperature over the
new surface-- so about two meters above the surface-- decreased polar ice sheets-- so that's
permanent glaciers on Antarctica and Greenland-- increased air temperature over the oceans,
marine air temperature, increased skin temperature, the sea surface temperature of the ocean, and
these two are the biggies.
So, an increase in ocean heat content, that's a measure of heat in the total ocean depth, so from
the surface down to about 3,000 metres, and increase in sea level. So while you see some shortterm volatilities in all these particular quantities, what we see in OHC and sea level is basically a
monotonic or consistent rise in the amount of heat accumulated there. And about 90% of the
additional energy from that 40% increase in CO2 is going into warming those oceans up. And at
some point, that will pass a threshold where it greatly impacts on the atmosphere that sits above
it.
I'll move on, now, to what this means for Australia. So it's important to acknowledge that the
attribution, so tagging these changes to greenhouse gases has been done very, very thoroughly.
That knowledge flows through to the changes at the local level. Having done the attribution at
the global scale, having looked at it in almost an epidemiological framework across different
continents, you can very strongly attribute a lot of changes that you see over Australia to the
same mechanism. There will be local micro changes that are due to things like land use changes,
stratospheric ozone changes impacting Australia, but it's the increase in greenhouse gases the
causes the big signal amongst all that noise.
So what's happened in Australia? So out of the report, Australia's climate has warmed, it's
warmed by about 0.9 of a degree, overall. The warming has been greater at night than during the
day, and that's actually a classic greenhouse gas signal, when you warm up the nighttime more
than the daytime, that's telling you that the atmosphere is retaining more moisture rather than the
sun heating up. And we've seen a change in our weather extremes with particular temperature
extremes with more extreme heat and less extreme cold.
So this is the temperature trend across Australia, that's the temperature change from 1910. And
the bureau was formed in 1908, and while we had some good, calibrated instruments around this
part of the country, it wasn't until 1910 that we distributed standardised, calibrated equipment
across the whole country. We have some reconstructions of temperatures going back to the mid
1880s through this region, but mostly we report from 1910. And as you can see, everywhere's
warmed, slight cooling in summer, here, due to increased rainfall, but otherwise, all territories,
all states, in all seasons have warmed up.
This is what that looks like as a time series. So these are anomalies with respect to a
climatological reference period from '61 to '90, and the rate here is Australian near surface
temperature, and the green line, there, is sea surface temperature in the regions around Australia.
So when people come and say, instruments over land are corrupted, that doesn't explain why
we've had almost the exact change in temperature in the ocean surrounding Australia, the
hemisphere, and the globe in general. So basically the consistency check it's not just this, that
there's a whole lot of consistency checks, but what we're seeing is not a micro change, we're
seeing a systematic change in the climate system itself.
And so that's the main warming. So what we know, here, is that we've basically pushed the
climate system by about one degree. it doesn't sound like a lot, but when you average
temperature over a large region, it's a little bit like taking your core body temperature. A small
shift in your core body temperature is much more significant than a change in the temperature of
your earlobe, for example. So a change in the Australian average temperature or the global
average temperature is much more significant than a degree change in the temperature of
Canberra, for example. So that's what we're doing when we're collecting these larger averages
together. And that small shift actually pushes the extremes quite a distance, and that's what I'll
show next.
So some people call this the hero graph out of the report, I'd never heard that term before, but
apparently this the one that most people latched onto. And just to explain what this is we're
trying to take a measure of the spatial extent of the heat and the severity of the heat across the
Australian continent. So we've got good daily data, now, back to 1910, and what we do is to
create an all-Australian average each day. And then we select the top 1% of those daily
anomalies per month, basically.
So all these bars, here, represented a count of the number of times the national daily temperature
went into the top one percentile of heat instances around the country. So what this has to collect
is heat that is widespread across the continent and it's quite exceptional, so up to 12 degrees
above average in some instances. And what you can see here is quite clear, from 1910 to 1941,
we had about 29 days where the national temperature hit that top percentile.
In 2013, we had 28 days that hit that top percentile. And you can see 2009, 2005, these were all
record years, basically, and the incidence of heat, the frequency of heat, the spread and the extent
of that heat, and the intensity of the heat is increasing. And we see that in the heat wave statistics
as well. The heat waves are becoming longer, the central intensity of them are higher, and their
frequency is higher as well. So this is kind of collected together in this graphic as well.
Rob Vertessy, our director, showed this graph earlier today, I'm just going to show it in the
context of what we've just shown. Why do we get that large shift in extremes? So, these are
temperature bell curves, or normal distributions, from 104 stations distributed across Australia
where we have very good records for. And we've binned this into three epochs, so we've got
1951 to '80, 19-- I think that's-- '81 to 2010, and then a period that's the last 15 years, 1999 to
2003.
So this drift to the right is a heating up of temperatures, and the main shift is actually all this
binning here, so that's the same old warm weather from the 1950s, but just much more often. But
what we've got here is two standard deviations, which is what we're calling extremely warm
months. And you can see that months that occurred around 2% of the time in the 1950s through
the mid century, are now occurring about 10% of the time. So it's a fivefold increase in hot
months, and we've got about a third decline in cold months over that time.
So that's that nudge to the distribution that actually pushes you into extremes. This is the proof of
climate change. Unfortunately, what we feel is whether out here, so you'll talk to a lot of people
that are expecting to see temperatures they've never seen before as proof of climate change. With
a 40% increase in CO2, you're more concentrating on this shift in distribution.
And Rob also showed this plot, and that's 2013, it was Australia's warmest year on record. What
we've plotted, here, is the daily temperature anomaly for the month, compared to 1961 to '90.
And this is actually a really extraordinary plot, mostly for most years you'd expect to see a mix
of cool and warm conditions, but what we had in 2013 was a sequence of heat waves in summer
and what we call mild waves or warm waves during winter. And these were really quite
extensive, you can see how much of the continent they affected. And really, it's this absence of
cold weather that's really very remarkable.
When we talk about that shifting distribution, it's not just that hot end, it's this complete absence
of intervening cold outbreaks, particularly over the southern parts of the continent, that are really
notable. And again, harking back to the Peter Hannam's talk for those that saw it this morning,
the absolute impact of this in terms of the duration and intensity of heat wave, has large
implications for human health and infrastructure, but it's the timing of these events that makes a
huge impact on agriculture. So a heat wave in spring can knock 10% to 15% off a yield. You get
it at the wrong time in occurrence with frost, and then you have really big issues as well.
So in terms of phenology and other seasonality issues, this year is one that we should just keep in
your mind as we go through the talk. The report also covered rainfall changes. Somewhere in the
public psyche, rainfall, drought got associated with climate change. Actually, when you look at
the climate models around since 1990, a warmer atmosphere means more moisture in the
atmosphere and more rainfall. It also means more evaporation, and that translates to wet areas
getting wetter, and dry areas getting drier, and that's actually kind of what we've seen in
Australia. I've got a question mark, here, for this point, that rainfall average across Australia has
slightly increased since 1900 with the largest increase since 1970, and that question mark is just
due to that large background variability in Australian rainfall, makes it hard to discern trends
over time.
But we have discerned some changes, and that's what we're here to talk about. So this is a map of
summer temperature deciles, so blue, here, is very much above average, the highest on record
since '95, '96, over that summer monsoon period equal to October. And it's rainfall through here,
during these months, that's largely responsible for that increase in Australian rainfall.
And I've put a satellite map here, this is actually a satellite picture of an active monsoon, so
we've got these tropical systems bringing rainfall. So typically, all the weather is coming from
above that red line during this time of the year, it's not really originating down south. And it's
weather from these tropical systems that's increased the overall rainfall in Australia. But amid
that background of variability, whether this is a pattern that will continue into the future, even the
models aren't quite clear on that one.
What's probably more certain in the data is a drying trend across the south. So, particularly in the
southwest of WA, rainfall since 1970 has undergone a step change. And it's mostly in winter,
that's when they get most of their rainfall. And it's a step change of about 10% to 20% reduction
in that rainfall. And we've seen that across in the southeast since about 1990, we've got late
autumn and early winter rainfall, again, about a 15% decline at that time of year. And this is a
picture of that rainfall decline.
So, again, what I've got, here, is a rainfall decile map. Red, on this one, is lowest on record, or
very much below average in orange, and now we're talking about rainfall that's coming from
these colour flows and cold fronts that sweet across the continent during the cooler months of the
year. And what you can see, here, for the southwest is the rainfall here in winter was some of the
most reliable wintertime rainfall in the whole continent.
And actually, if you go out there and look at the ecology, it's very similar to mountain ash, the
[INAUDIBLE] forests. So there was obviously a high rainfall regime over this area of the
continent for a very, very long period of time. I've gone down into the limestone casts in this
region, which are the underground caves, and you can actually see the high watermarks
somewhere around your head.
So you can see both a loss of the very wet winters, and a reduction in rainfall there. And that
temperature and rainfall change has translated through to more fire weather. So I'll quickly touch
on this in the time I've got left. These big circles, here, are increases in both the cumulative fire
danger index and the extreme fire danger index. So it's a forest fire danger index, and what we've
seen is the top percentiles of that fire weather is becoming more extreme, and the fire weather is
pushing into autumn and spring, so we've go a longer fire season. That's Melbourne airport-- so
I'm going to just take a few more seconds-- Melbourne airport's in a great location for fire
activity in the southeast. And you can see the influence of the millennium drought, there, and the
two years of rainfall. But this signal is independent if that rainfall variability, it's driven by the
temperature change.
So, quickly finishing up, these are projections for Australia. What we've got here is around the
country, basically, northern Australian rainfall, quite uncertain, but southern Australia, our
projection is the continued drying. In fact, it's a part of the world where the models really,
closely agree on rainfall declines, particularly in the southwest, which means increased
frequency and severity of drought, more extreme fire weather, a prolonged fire season, higher
sea levels, an ocean acidification.
So I've got a way I show this to audiences, because I think projections don't mean much at all. So
what I do is I take a thought experiment, which is, there's a landscape analogue out there for the
climate change that your region is going to experience, and there's probably someone using that
land quite productively right now. So if I take a location in central Victoria, so the Yarra Valley
There's the Yarra Valley in drought and in good times, and I make it two degrees warmer or 20%
drier, I shift it somewhere into the [INAUDIBLE], this is Horsham during drought and good
times, and you can see it's a very different climatology. And this is the way climate change
would be expected to move across the landscape.
I can do that again by taking more marginal country, so it's starting in Horsham, and then taking
that 20% dryer and about two degrees warmer, and you're now talking about the Central Darwin,
and that's really quite a different location, again. The problem is that this is going to occur
simultaneously across the whole continent. And my last slide is the one that Rob showed, and it
doesn't show up that well on this, but what I've got here, is about 28 different climate models
forced with greenhouse gases only from around the world for Australian annual temperature, and
that white line you can just see, there, is the actual observed temperature.
What we had in 2013 was the warmest year on record. It's continental-wide heat, as discussed
earlier. So in terms of that implication going forward, by about 2030, that record warm year sits
in the mid of the distribution, so it's about an average year. And then sometime by the end of the
century, that becomes a really unusually cold year. So in terms of adaptation, you can probably
wait a bit to adapt, you've got to see the lay of the land, you've got to see what's happening.
There are adaptation measures, but I guess this is the global mitigation problem. Can you adapt
to what's going to happen at this end of the spectrum? Thank you.