Wednesday, December 23, 2015

Blog Break

After five years of nearly continuous blogging, I am taking a break during the holidays this year.  Please check back in 2016.  My best wishes to you for deep powder and an enjoyable holiday season.

If you enjoy this blog, please consider making a donation to our Mountain Meteorology Fund.  The fund supports research and education in mountain weather and climate at the University of Utah.  Visits by people like Jordy Hendrix were made possible this past year because of the fund, as well as support for students who make many of the custom and experimental products we provide on and which are used by many in the Wasatch forecast and ski communities.

A Recipe for Upside Down Cake

Skiing today involved breaking trench up...and at times, down!
After reading some reports of good low angle skiing yesterday, I was pumped for a ski tour today. In the wake of this morning's cold front, however, temperatures dropped and the winds picked up.  Peak gusts on the higher peaks exceeded 80 mph and snow was moving nearly everywhere, including at lower elevations. Basically it was a perfect recipe for an upside-down-cake snowpack with the high density wind affected snow sitting on top of a remarkably deep layer of lower density snow.  Conditions were about as difficulty as any I've skied in open areas.  My advice for tomorrow: Stick to the low angle trees in wind sheltered areas and hope for the best!

The Big Numbers So Far

It's a hell of a storm and it is raging again this morning as a cold from moves through northern Utah making the morning commute miserable in the Salt Lake Valley, but also pushing accumulations in the mountains to even higher levels.  It should be a quick hitter for the valley, but the timing is bad news for anyone having to travel during the morning rush.
Source: RAL
As of this morning, water totals in the mountains are impressive.  Ben Lomond Peak's automated gauge is up to nearly 5 inches (green dots).  Snowbasin middle bowl is approaching 4.5 inches (blue line).  Alta-Collins is at 2.4 inches.  The big winner is Sundances site with over 6.5 inches, although the Utah Avalanche Center is reporting 4 inches in their advisory this morning, so I wonder if that might be overdone compared to manual observations.  It is apparent, however, that they got quite a pasting down there.

Source: MesoWest

With the total snow depth at Alta-Collins up to 63", I'm officially announcing the start of "good early season conditions" (60 inches is my threshold for that).  

Tuesday, December 22, 2015

Alta Isn't Always King

It is unusual, but upper Little Cottonwood seems to be a dry spot in this storm, although dry spot in this sense is a relative term.

Observations from Alta-Collins show a total of about 27 inches of snow if you add up all the interval accumulations before the board is wiped back down to zero (top panel) and include the 4 inches observed so far today.  The water total is only 1.6 inches, although there may be some underdone a bit as that yields a water content of only 6%, which seems quite low compared to other locations.

Source: MesoWest
In contrast, Snowbasin has had 3.4" of water, more than double that at Alta-Collins (note scale change).

Source: MesoWest
In addition, snowfall reports from upper Big Cottonwood and Deer Valley have also topped those in upper Little Cottonwood.  I'm less sure about PCMR, but suspect they are at least close to what Alta has had so far too.  

And the latest radar shows that it continues to snow at Snowbasin, but Alta is temporarily out of the echoes.  

Factors contributing to the lower Alta snow totals in this event are multiple and include the predominantly southerly or southwesterly flow yesterday (favoring the northern Wasatch and yesterday afternoon upper BCC and Deer Valley), a tendency for the strongest precipitation features to be to our north (e.g., radar above), and just plain dumb luck (sometimes even Alta misses out on the more scattered and chaotic snow showers). 

No need to shed any tears as 27" in 48 is still just fine and eventually the law of averages will win out.  

Santa Delivers Something for Everyone

We may be amateurs, but every now and then we pull the government weather-control levers and get it right.  Despite a brief lull yesterday evening that briefly caused many of my friends to text or e-mail me to ask if the storm was over (no, it wasn't), this storm has has provided Christmas presents for all you girls and boys. Let's start with some water totals since 5 AM yesterday from automated sensors that report to MesoWest:

Northern Wasatch
Ben Lomond Peak: 3.3" (through 6 am)
Ben Lomond Trail: 2.1" (through 6 am)
Snowbasin Middle Bowl: 2.35" (through 7:15 am)

Central Wasatch
Alta-Collins: 1.01" (through 7 am)

I'm seeing some widely disparate SNOTEL measurements in the central and southern Wasatch depending on if the gauge or pillow measurement is used, so I'm not using them here.  Snow reports in Park City and Big Cottonwood Canyon, however, are much higher than those in Little Cottonwood Canyon, so it's safe to assume that Alta-Collins is one of the drier spots in the storm so far and that higher water totals were observed elsewhere.  Ditto for the southern Wasatch where the Utah Avalanche Center has raised the avalanche danger to extreme with 3.7" of water equivalent falling at 7500 feet in the mountains near Provo.  Given that Alta and Snowbird have been on top in most every storm so far this year, it seems only fair that socialist Santa spreads the wealth a little bit in this storm.

The latest radar loop [1114-1432 UTC (0414-0732 MST)] shows a nearly solid wall of returns upstream of the Wasatch Front with precipitation enhancement along much of the Wasatch Range in the now west to west-northwest crest-level flow.  

Snow levels have dropped to the valley floor.  I had a white knuckle bus ride down from the Avenues this morning as the plows hadn't been out yet and roads were snow covered with that slick snot that you get after the transition from rain to snow.  Kudos to my UTA driver for an expert descent down Virginia Street!

The models keep us in a fairly moist west to west-northwest crest-level flow through tomorrow afternoon.  Although there could be a break or two in there, it appears accumulations in the mountains will continue to be substantial.  Further, the valleys are going to see accumulations as well, ensuring a White Christmas for nearly everyone, unless someone gets skunked in a localized precipitation shadow (although that's looking unlikely).  

Let's start again with the experimental high-resolution NCAR ensemble.  Accumulations below are from 0000 UTC (1700 MST) yesterday afternoon through 0000 UTC (1700 MST) tomorrow afternoon, so keep in mind that these numbers do include last night's forecast.  Mean water totals in the central Wasatch exceed 2 inches and in the northern Wasatch are generally greater than 1.5 inches.  

The forecast plume for Snowbasin below shows the heavier precipitation last night, after which accumulation rates back off a bit through tomorrow afternoon.  From about 1200 UTC (0500 MST) this morning to 0000 UTC (1700 MST) tomorrow afternoon the total accumulation is about an inch for most members.  

At Alta Collins, the total accumulation from 1200 UTC (0500 MST) this morning to 0000 UTC (1700 MST) tomorrow afternoon ranges from 0.8 to 1.7" depending on the member.  

Given the predominantly westerly to northwesterly flow, the base of Park City probably won't do as well as yesterday, but they will get some.  The total accumulation from 1200 UTC (0500 MST) this morning to 0000 UTC (1700 MST) tomorrow afternoon is 0.4" to perhaps 0.7".  

I might drop the numbers for Snowbasin and Alta-Collins just a bit, but by and large, these water numbers look to be a pretty good hack.  For the period from 1200 UTC (0500 MST) today through 0000 UTC (1700 MST) tomorrow afternoon, let's go with 0.75-1.25 inches of water for the northern Wasatch and 1-2 inches of water for the upper Cottonwoods.  In town, Park City won't do as well as yesterday, but you'll still get some, although clearly Santa's socialist streak is over.

For the valley?  Ah, you're on your own for that.  I don't have the time!

Monday, December 21, 2015

Let's Get This On

I love mornings like this as I anticipate an approaching storm and peer out of my office at a cap cloud draped over Lone Peak.

We had a pretty good storm last week, but that was a cold storm producing low density cold smoke.  This week's storm promises to be of the juicier variety.

This morning's 1200 UTC (0500 MST) analysis shows a potent atmospheric river, characterized by a tongue of high precipitable water (warm colors) extending from the sub-tropical west Pacific across the north Pacific.

Atmospheric rivers typically decay rapidly as they move inland, so if you one to survive intact to Utah, it's best to start with a strong one on the coast and then to bring it inland through regions of low topography so that there's less water-vapor depletion over upstream mountain ranges.  Avoiding the southern high Sierra is essential, and the best option is to bring it across the comparatively low Sierra and Cascade crest of northern California and across northern Nevada and southern Idaho where the topography is less continuous and formidable.

The forecasts below are valid for this 0000 UTC (1700 MST) this afternoon and show the penetration of the atmospheric river through that hallowed weak terrain corridor and into northern Utah.  The area outlined by the red contour denotes integrated water vapor transport values exceeding those typically used for identifying atmospheric river conditions (i.e., 250 km/m/s).

Source: NWS
Such values are not exceptional for northern Utah, but for the 3-week period centered on today, they have a return interval of about once per year and perhaps once every 5 years in the core of the atmospheric river.

Source: NWS
The time-height section at Salt Lake City from this morning's NAM is unlike anything we've seen so far this season.  Note the deep layer of moisture from 1800 UTC Monday through 1800 UTC Tuesday.  The flow at low levels is southerly to southeasterly southwesterly, whereas at crest level the flow is initially southwesterly and transitions to WNW during the period.  There is also weak stability tonight.
Overall, this is a pattern in which the northern Wasatch should do fairly well and I'd expect to see some healthy totals from Ben Lomond Peak and Snowbasin.  The depth of the storm will also help with spillover to the Wasatch Back.  The westerly flow can sometimes be less favorable for the Cottonwoods, but this is a juicy enough storm that probably won't matter much.

The plots below are from the experimental high-resolution NCAR ensemble and are for the 24-hour period from 0000 UTC (1700 MST) yesterday through 0000 UTC (1700 MST) Tuesday, although most of the precipitation falls during the aforementioned 1800 UTC Monday through 1800 UTC Tuesday period.  Ensemble mean water totals are above 3 inches across most of the high terrain of the Wasatch.
Let's look at a few locations.  At Alta-Collins, water totals if we adjust for the small amounts last night, range from 2-3.5 inches through tomorrow afternoon.  Note that I think the storm is coming in a bit too late in this ensemble and is likely to get going in earnest earlier than suggested.

Ben Lomond Peak can often be a big winner in a pattern like this and the numbers there go a bit higher, including one member going for nearly 6 inches for the full 48-hour period.  The latest model runs shift the storm southward a bit more quickly, so I'm not sure we'll be able to get to those numbers, but Ben Lomond has a habit of going big quickly in storms like this.

And the base of Park City the model is going for 1.5 to 2.5 inches of water, which would be a godsend for them.

This will be a warm storm initially, but temperatures are low enough that all of the precipitation at these sites should fall as snow.

We don't know a lot about how the experimental NCAR ensemble biases, so at issue is how much should we believe these numbers.  For comparison, the 12-km NAM generates 2.21" of water at Alta-Collins from 1200 UTC (0500 MST) this morning through 0000 UTC (1700 MST) tomorrow, which is about as big of a number as you'll see that model generate.

If we look at long-term statistics for Alta-Collins, events that generate more than two inches of water in 24 hours (plot below based on observations from 4 am to 4 am) happen perhaps 3 times per year, but more than 3 inches of water are pretty exceptional (about once every 10 years based on the graph below).

So, let's put some numbers on this.  For the period beginning at 1200 UTC (0500 MST) this morning through 0000 UTC (1700 MST) tomorrow afternoon, let's go with 2-4 inches of water at upper elevations in the northern Wasatch and 1.5-3 inches in the central Wasatch.  There is potential for locally higher amounts.  These numbers are a bit lower than those generated by the NCAR ensemble, but I've seen that ensemble overgenerate precipitation in similar situations in the Pacific Northwest and am not certain there are enough ingredients here to go for an event that would be on the tail of the  climatological distribution.  

Snow density will probably be high, especially today, bug decrease during the storm.  Something in the 1.5 to 3 foot range seems reasonable, with the potential for locally higher amounts.

Bottom line: Things look pretty good for a major event.  I've been hoping to sneak out for a ski tour Tuesday or Wednesday, but this is the kind of storm that will make steep terrain too dangerous and keeping momentum going on low-angle slopes difficult.  If we had a healthy snowpack, I'd be rooting for a smaller storm, but we don't, so bring it on.

Saturday, December 19, 2015

Batten Down the Hatches

The Snow Miser has some tricks up his sleeve for Christmas Week
After some periods of mountain snow Sunday and Monday, some serious winter weather will be blowing into the Wasatch late Monday and Tuesday.  As shown by the GFS forecast below, which is valid for 1800 UTC (1100 MST) Tuesday, the Pacific jet is expected to extend into the Intermountain West, yielding strong westerly and west-northwesterly flow at nearly all levels.  This is an extremely moist fetch as well, with strong orographic enhancement expected over the mountains.

This can also be seen in the GFS time-height section for Salt Lake City (time increases to the left).  That period around 1800 UTC Sunday features deep moisture and very strong flow, reaching 50 knots as low as 750 mb (~8500 ft).  

Our downscaled NAEFS forecast plumes show the light accumulations ahead of the storm, but then the big event on Tuesday (22 December) when most ensemble members are generating over 2 inches of snow-water equivalent from 0000 UTC 22 Dec to 0000 UTC 23 Dec, with some more tacked on thereafter.  These are some of the largest water totals I've seen produced by this product since we started producing it last year, although they might be overdone a bit.  There are a couple of ensemble members that are a bit less bullish, so perhaps a huge water event isn't in the bag yet, but a significant storm is likely.  

This should be a "batten down the hatches" with both heavy snowfall and strong winds.  Weak layers in the snowpack should be stressed severely.  A reminder of the monsters lurking in the basement was provided to us yesterday as we gazed into Mineral Basin from Sugarloaf Pass.  

Friday, December 18, 2015

Friday Photos

Great banner cloud downstream of Mt. Baldy this morning.

Classes at the U ended last week, but these motivated graduate students got a real-world education about Wasatch weather today.

And what a sunset!

Thursday, December 17, 2015

Bluebird Morning, Cold Smoke, a Snowy Future, and Unreliable Social Media Forecasts

Clouds are quickly returning this morning over the Salt Lake Valley, but as is evident in the photo above, it was a bluebird sunrise that ignites a skier's soul this morning.  Winter has finally returned to northern Utah.

Cold Smoke

Although it took a little while to get going with the benches picking up more snow on Monday, the Little Cottonwood snowmaking machine has been going full bore since Monday night with another 12" falling overnight.  That brings their storm total to 46 inches of cold smoke.  The water content of that snow was only 1.9", yielding a mean water content of only 4.1%.  Thus, that 46" won't go as far as you think for base building, but have no fear, because more snow is coming....

A Snowy Future

Readers of this blog know I don't like to hang my hat on detailed, specific, medium-range forecasts, but by and large, it looks pretty good that after a bit of a break through Saturday (other than perhaps some snow showers tonight), we're going to see the snows return later in the weekend and early next week.  There are differences in the timing and intensity, but all members of our downscaled NAEFS ensemble are generating more than 2.5 inches of water equivalent through 0000 UTC 24 December at Alta-Collins.  I'm not sure whether or not to believe the high numbers being projected, but it's quite likely that we're going to add to the snowpack.

Avalanche conditions are already dicey (see this important post by UAC forecaster Brett Kobernik), but next week the backcountry could come unglued as the overall pattern is both wet and windy.  We'll see how things shake out in the coming days.

Unreliable Forecasts

I was pretty shocked yesterday to see the facebook post below from the Salt Lake Tribune, which called for 6-12 inches overnight last night.

I went back through the National Weather Service forecasts that were available at the time and could find nothing to justify such a posting.  In fact, the forecast for the Salt Lake Valley that would have been available at the time of the post above called for an accumulation of "around 1 inch."  That appears to have verified nicely, at least at my house.

Forecasting is hard and we're going to have some false alarms and underpredictions, so erroneous social media reports like the one above really bother me as they erode public confidence.  The National Weather Service isn't perfect, but they provide reliable, unsensationalized, timely forecasts for northern Utah at

Tuesday, December 15, 2015

Storm Perspectives

It's been quite a while since we had a storm like this, especially on the valleys and benches.  It's great to have winter as it should be.  A few thoughts on the event this morning.

How good were the forecasts?

Snow forecasting in northern Utah is all about synoptic possibilities and mesoscale uncertainties.  The former means that we can often anticipate when the large-scale ingredients for a significant storm are present, but the latter implies that the devil is in the detail and specific forecasts of the timing, intensity, and spatial distribution of the snowfall is exceedingly difficult.

For the present storm, we had strong agreement amongst the models and ensemble modeling systems that a significant storm would hit the state.  The NWS issued watches and warnings in a timely manner with good lead time.  The forecasts did not capture, however, some aspects of the snowfall timing, intensity, and distribution, most notably in some bench areas where the snow came harder and faster yesterday than anticipated, resulting in greater accumulations than forecast.  In addition, their briefing on Sunday afternoon suggested that the Salt Lake Valley may see less of an impact for the Monday morning commute (emphasis on may), although my small sample of Sunday evening news broadcasts didn't seem to give much of a hint of this.

Forecast updates yesterday afternoon for last night also appear to have overdone the overnight accumulations, at least in the Salt Lake Valley.  Nevertheless, given the limitations of the science today, by and large the forecasts were generally good for this event.

Failure to prepare is preparing to fail

Yesterday morning's rush hour was a mess.  The first major storm of the year is always bad, but really this was the first major storm in two years, and it showed.  A few thoughts come to mind.  First, it appears we've forgotten what snow looks like.  Yes, that white stuff on the road really is snow and it does pay to slow down when driving on it.  Second, the bald tires we've been putting off replacing now for two years really don't work in the snow.  Finally, when our tires slip, it can be satisfying to push the accelerator all the way to the floor, but that really doesn't do any good.  Of course, these statements don't apply to any of us as I'm sure all readers of this blog were well prepared :-).

What surprised me most about this event was how unprepared the U and many school districts appeared to be.  Given the forecasts, I didn't expect that.  The contrast in the information feed from yesterday morning to this morning is quite large and suggests that everyone was at best out of practice and at worst simply did not take the forecasts seriously.  Yesterday I received no notices from the U during the morning rush hour and when I finally did get one, it came via a forwarded e-mail from our administrative assistant because the system was apparently overloaded.  Today, I've already received a notice concerning campus status.  Much better.  Ditto for West High where my children attend.  No info until late yesterday morning, but notices are flying in via text and voice mail today.

I do feel for those who have to provide these notices and make closure decisions.  Forecast reliability for these events isn't as high as we would like, and there are a significant number of false alarms for major snow events.  On the other hand, yesterday seemed like a case of being unprepared.  Hopefully we've gotten that out of our systems and we're ready to go for the rest of winter.

Stuff I don't understand

The enhancement of precipitation on the eastern benches of the Skull, Tooele, and Salt Lake Valleys (as well as the Bountiful Bench) was most impressive and produced precipitation rates (snowfall and water equivalent) that were much higher than observed in the higher mountains.  Although I have hypotheses for why this happens, I don't really understand why.  Further confusion is added by the overnight transition to higher precipitation rates at Alta-Collins, which (fortunately) yielded another 14 inches of snow and .45" of water after 6 PM.

We don't understand these variations in orographic enhancement and we certainly can't forecast them.  Some of you people out there who are smarter than me should help us figure this out.

Monday, December 14, 2015

Remarkable Shallow Orographic Effects

Don't be fooled by the relatively modest water equivalent rates in the upper elevations of the Cottonwoods today. The mountains are having a dramatic influence on this storm, but more in the lower elevations.

The word orographic means relating to mountains.  When we are talking about orographic precipitation or orographic effects, we mean precipitation or effects related to the mountains.

The orographic effects today are dramatic and pronounced on the radar.  Check out the radar loop below, which covers the period from 1938–2144 UTC (1238–1444 MST).  In the northwesterly flow, there is clear enhancement of precipitation on the windward side of the Cedar, Stansbury, Oquirrh, and Wasatch Ranges as one moves from west to east across roughly the center of the image.

Note also that there's shadowing on the east (downstream) side of those ranges too.

If one looks carefully over the Stansbury, Oquirrh, and Wasatch ranges, however, the highest returns are not found over the crest but over and upstream of the windward slopes.  Totals in the lower Cottonwood Canyons, for example, will be larger for this part of the storm than in the upper Cottonwood Canyons.  What a pity!

Upcoming Talks

There are a couple of good opportunities to learn about Wasatch weather and climate this week.  Surely you are interested after todays storm?

At 7:30 PM on Wednesday evening, UAC Forecaster Drew Hardesty and University of Utah Professor and fellow Wasatch Weather Weenie Larry Dunn will be giving a fireside chat entitled "Predict the Pow: Backcountry Meteorology 101" at the Black Diamond Retail Store.

Source: Black Diamond and Utah Avalanche Center
At 6:00 PM on Thursday evening, I'll be giving a talk on my book Secrets of the Greatest Snow on Earth, with an extended section on the potential impacts of global warming on Wasatch skiing, for the Utah Adventure Journal Speaker Series at the Wildflower Lounge in Snowbird's Iron Blosam Lodge.  Admission is free, but you must be 21 to attend.  Consider making a $5 (or more) donation to the Utah Avalanche Center, which gets you into the drawing for some sponsor schwag.  

Iconic Moment at the U!

A few members of the University of Utah Cross Country Team were out training today.  Love it!

The Benches Are Getting More Snow Than the Mountains

For the overnight and morning stages of this storm, the benches have gotten more snow than the central Wasatch Mountains.  This is true for both total snowfall and water equivalent.  For example, as of 10 am we've had 0.95" of water equivalent at our observing site here at the University of Utah campus, whereas only 0.47" has fallen at Alta-Collins.

We don't completely understand why we get these upside down storm periods, but there are some common threads.  First, they nearly always feature light, shallow, northwesterly flow.  Below is a radar image from earlier this morning showing surface observations with predominantly W or NW flow.  I've highlighted three boxes.  The westernmost box is on the west side of the Oquirrhs.  Note the precipitation enhancement there.  The central box is over the east bench of the Salt Lake Valley.  Note the precipitation enhancement there as well (plus further north along the northern Wasatch Front.  Finally, the easternmost box is over the central Wasatch.  Note the weak returns there.  

Just to help with orientation, here's a map without the radar.

Likely critical to the distribution of flow in this event is the shallow nature of the northwesterly flow.  In the Salt Lake Valley, winds were light but northwesterly since just before midnight MDT and picked up this morning just before 5 AM.  Obs below from the Spaghetti Bowl in the center of the valley.

In contrast, the flow on Mount Baldy was southerly all night long and did not shift to westerly until 7 am.  Even now, the flow on Mt. Baldy is only 18 mph, which is very weak for that location.

These sorts of storm characteristics are much easier to sort out in hindsight and very difficult to anticipate in a forecast.   These local-scale precipitation effects are highly sensitive to small changes in wind direction, wind speed, and vertical wind shear.  As is often the case in northern Utah, winter storms are about synoptic probabilities, but what you experience depends on smaller-scale processes that are very difficult to predict several hours or more in advance.  Clearly we have more work to do.

Storm Update @ 7:50 AM

As expected, a winter storm moved into northern Utah overnight and is giving us the best storm of the season so far.  Based on reports to the National Weather Service, there are some impressive snow totals on the benches including 13 inches on the Bountiful Bench (as of 6 am), 12 inches in Cottonwood Heights (as of 6 am), 7 inches in the upper Avenues (as of 6 am).  In the mountains, Snowbasin Middle Bowl was at 10 inches as of 5 am and Alta-Collins sits at 7 inches as of 7 am.

I suspect the snow at Snowbasin was heaviest during the southwesterly crest-level flow overnight. The heavy bench snow, however, is very consistent with what we've seen on radar since the low-level flow shifted to northwesterly.  Note how the highest returns are on the benches west of the Stansbury, Oquirrh, and Wasatch Mountains, especially in the terrain concavities near Bountiful/North Salt Lake and the mouths of the Cottonwoods.

Mountain precipitation rates haven't been as great in the central Wasatch, as one might infer from the radar above.  The flow right now is fairly weak and the orographic enhancement is confined to the benches.  Although 7" has fallen at Alta-Collins, there's only been 0.39" of water with it.

Sunday, December 13, 2015

Expect "Tough Sledding" Tomorrow

I've been out skiing and lack the time to put together a proper forecast for tomorrow.   Below is the winter storm watch issued early this morning by the National Weather Service.  This is going to be a statewide storm with significant travel impacts.  Plan on a rough commute in the morning, and hope for the best.

The NWS forecast for the mountains looks pretty good as well.  This will probably be the biggest storm of the season so far (although that's a fairly low bar, with water totals somewhere in the 1-2 inch range by 5 PM Tuesday.

Enjoy the storm and be careful out there.

Saturday, December 12, 2015

The Paris Agreement

"New agreement is just words.  We're still on a business-as-usual course."
- Jim Hansen

Today representatives of 195 nations approved an agreement that commits every country to reduce greenhouse gas emissions.  The full document, available here, recognizes that climate change "represents and urgent and potentially irreversible threat to human societies" and that "deep reductions in global [greenhouse gas] emissions will be required."  The agreement seeks to hold the global average temperature "to well below 2ºC above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5ºC above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change."  

In case you are wondering we are currently about 0.8ºC above pre-industrial levels [2015 is a full 1ºC above pre-industrial levels, although that's partly due to the intense El Nino], with additional warming already in the pipeline as the climate system has not yet fully adjusted to the rapid increase in greenhouse gas concentrations over the past few decades.  

I've been asked a few times about my views on the Paris Agreement (and before this month, many times about Kyoto before it).  I'm pretty much a political cynic and so my perspectives are similar to Jim Hansen's above.  I am concerned about global warming, which reflects my scientific understanding but also personal values, and thus am glad to see the agreement recognize the pressing need to address greenhouse gas emissions.  My political cynicism is such that I ultimately see agreements like this as something only diplomats could love.  When the rubber hits the road tomorrow, we're still part of a global society in which the long-term greenhouse gas emissions trend is upward.  There's some indication that global greenhouse gas emissions in 2015 will decline slightly from 2014, but we have to reduce global greenhouse gas emissions by about 40% just to get back to 1990 levels.   

Source: Global Carbon Project
At the same time, we need to nearly quadruple our global energy production to bring the developing world up to a reasonable standard of living.  

It has been said that a journey of 1000 miles begins with a single step, and an optimistic view of the Paris Agreement is that it makes the first step on that journey.  However, we shouldn't lose sight of what ultimately needs to be done.  Incremental improvements in efficiency and lifestyle changes such as riding bikes and driving hybrid cars are necessary are good, but they are not sufficient to meet the long term need for carbon neutrality.  As Richard Smalley, the 1996 Nobel Laureate in Chemistry once wrote, we ultimately need "cheap, clean energy in vast amounts."  In other words, innovation is needed to make a quantum leap in how we produce, distribute, and store energy.  

That innovation needs to be priority one today (and unfortunately should have been a priority many years ago).