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Sun & climate: moving in opposite directions

What the science says...

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The sun's energy has decreased since the 1980s but the Earth keeps warming faster than before.

Climate Myth...

It's the sun

"Over the past few hundred years, there has been a steady increase in the numbers of sunspots, at the time when the Earth has been getting warmer. The data suggests solar activity is influencing the global climate causing the world to get warmer." (BBC)

At a glance

Thankfully for us, our Sun is a very average kind of star. That means it behaves stably over billions of years, steadily consuming its hydrogen fuel in the nuclear reaction that produces sunshine.

Solar stability, along with the Greenhouse Effect, combine to give our planet a habitable range of surface temperatures. In contrast, less stable stars can vary a lot in their radiation output. That lack of stability can prevent life, as we know it, from evolving on any planets that might orbit such stars.

That the Sun is a stable type of star is clearly demonstrated by the amount of Solar energy reaching Earth's average orbital position: it varies very little at all. This quantity, called the Total Solar Irradiance, has been measured for around forty years with high accuracy by sensitive instruments aboard satellites. Its average value is 1,362 watts per square metre. Irradiance fluctuates by about a watt either way, depending on where we are within the 11-year long sunspot cycle. That's a variation of no more than 0.15%.

From the early 1970s until today, the Solar radiation reaching the top of Earth's atmosphere has in fact shown a very slight decline. Through that same period, global temperatures have continued to increase. The two data records, incoming Solar energy and global temperature, have diverged. That means they have gone in opposite directions. If incoming Solar energy has decreased while the Earth continues to warm up, the Sun cannot be the control-knob of that warming.

Attempts to blame the sun for the rise in global temperatures have had to involve taking the data but selecting only the time periods that support such an argument. The remaining parts of the information - showing that divergence - have had to be ditched. Proper science study requires that all the available data be considered, not just a part of it. This particular sin is known as “cherry-picking”.

Please use this form to provide feedback about this new "At a glance" section, which was updated on May 27, 2023 to improve its readability. Read a more technical version below or dig deeper via the tabs above!


Further details

Our Sun is an average-sized main sequence star that is steadily using its hydrogen fuel, situated some 150 million kilometres away from Earth. That distance was first determined (with a small error) by a time consuming and complex set of measurements in the late 1700s. It led to the first systemic considerations of Earth's climate by Joseph Fourier in the 1820s. Fourier's number-crunching led him to realise a planet of Earth's size situated that far from the Sun ought to be significantly colder than it was. He was thereby laying the foundation stone for the line of enquiry that led after a few decades to the discovery of what we now call the Greenhouse Effect – and the way that effect changes in intensity as a response to rising or falling levels of the various greenhouse gases.

TSI Solar cycles

Figure 1: Plot of the observational record (1979-2022) on the scale of the TSIS-1 instrument currently flying on the space station. In this plot, the different records are all cross calibrated to the TSIS-1 absolute scale (e.g., the TSIS1-absolute scale is 0.858 W/m^2 higher than the SORCE absolute scale) so the variability of TSI in this plot is considered to be its “true variability” (within cross calibration uncertainties). Image: Judith Lean.

The Sun has a strong magnetic field, but one that is constantly on the move, to the extent that around every 11 years or so, Solar polarity flips: north becomes south, until another 11 years has passed when it flips back again. These Solar Cycles affect what happens at the surface of the Sun, such as the sunspots caused by those magnetic fields. Each cycle starts at Solar Minimum with very few or no sunspots, then rises mid-cycle towards Solar Maximum, where sunspots are numerous, before falling back towards the end. The total radiation emitted by the Sun – total solar irradiance (TSI) is the technical term – essentially defined as the solar flux at the Earth's orbital radius, fluctuates through this 11-year cycle by up to 0.15% between maximum and minimum.

Such short term and small fluctuations in TSI do not have a strong long term influence on Earth's climate: they are not large enough and as it's a cycle, they essentially cancel one another out. Over the longer term, more sustained changes in TSI over centuries are more important. This is why such information is included, along with other natural and human-driven influences, when running climate models, to ask them, “what if?"

An examination of the past 1150 years found temperatures to have closely matched solar activity for much of that time (Usoskin et al. 2005). But also for much of that time, greenhouse gas concentrations hardly varied at all. This led the study to conclude, "...so that at least this most recent warming episode must have another source."

TSI vs. T
Figure 2: Annual global temperature change (thin light red) with 11 year moving average of temperature (thick dark red). Temperature from NASA GISS. Annual Total Solar Irradiance (thin light blue) with 11 year moving average of TSI (thick dark blue). TSI from 1880 to 1978 from Krivova et al. 2007. TSI from 1979 to 2015 from the World Radiation Center (see their PMOD index page for data updates). Plots of the most recent solar irradiance can be found at the Laboratory for Atmospheric and Space Physics LISIRD site.

The slight decline in Solar activity after 1975 was picked up through a number of independent measurements, so is definitely real. Over the last 45 years of global warming, Solar activity and global temperature have therefore been steadily diverging. In fact, an analysis of solar trends concluded that the sun has actually contributed a slight cooling influence into the mix that has driven global temperature through recent decades (Lockwood, 2008), but the massive increase in carbon-based greenhouse gases is the main forcing agent at present.

Other studies tend to agree. Foster & Rahmstorf (2011) used multiple linear regression to quantify and remove the effects of the El Niño Southern Oscillation (ENSO) and solar and volcanic activity from the surface and lower troposphere temperature data.  They found that from 1979 to 2010, solar activity had a very slight cooling effect of between -0.014 and -0.023°C per decade, depending on the data set. A more recent graphic, from the IPCC AR6, shows these trends to have continued.

AR6 WGI SPM Figure 1 Panel p

Figure 3: Figure SPM.1 (IPCC AR6 WGI SPM) - History of global temperature change and causes of recent warming panel (b). Changes in global surface temperature over the past 170 years (black line) relative to 1850–1900 and annually averaged, compared to Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model simulations (see Box SPM.1) of the temperature response to both human and natural drivers (brown) and to only natural drivers (solar and volcanic activity, green). For the full image and caption please click here or on the image.

Like Foster & Rahmstorf, Lean & Rind (2008) performed a multiple linear regression on the temperature data, and found that while solar activity can account for about 11% of the global warming from 1889 to 2006, it can only account for 1.6% of the warming from 1955 to 2005, and had a slight cooling effect (-0.004°C per decade) from 1979 to 2005.

Finally, physics does not support the claim that changes in TSI drive current climate change. If that claim had any credence, we would not expect to see the current situation, in which Earth's lower atmosphere is warming strongly whereas the upper atmosphere is cooling. That is exactly the pattern predicted by physics, in our situation where we have overloaded Earth's atmosphere with greenhouse gases. If warming was solely down to the Sun, we would expect the opposite pattern. In fact, the only way to propagate this myth nowadays involves cherry-picking everything prior to 1975 and completely disregarding all the more recent data. That's simply not science.

Longer-term variations in TSI received by Earth

It's also important to mention variations in TSI driven not by Solar energy output but by variations in Earth's orbit, that are of course independent of Solar activity. Such variations, however, take place over very long periods, described by the Milankovitch orbital cycles operating over tens of thousands of years. Those cycles determine the distance between Earth and the Sun at perihelion and aphelion and in addition the tilt the planet's axis of rotation: both affect how much heat-radiation the planet receives at the top of its atmosphere through time. But such fluctuations are nothing like the rapid changes we see in the weather, such as the difference between a sunny day and a cloudy one. The long time-factor ensures that.

Another even more obscure approach used to claim, "it's the sun" was (and probably still is in some quarters) to talk about, "indirect effects". To wit, when studies can't find a sufficiently large direct effect, bring even lesser factors to the fore, such as cosmic rays. Fail.

In conclusion, the recent, post 1975 steep rise in global temperatures are not reflected in TSI changes that have in fact exerted a slight cooling influence. Milankovitch cycles that operate over vastly bigger time-scales simply don't work quickly enough to change climate drastically over a few decades. Instead, the enormous rise in greenhouse gas concentrations over the same period is the primary forcing-agent. The physics predicted what is now being observed.

Last updated on 27 May 2023 by John Mason. View Archives

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Further viewing

This video created by Andy Redwood in May 2020 is an interesting and creative interpretation of this rebuttal:

Myth Deconstruction

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MD Sun

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Expert interview with Mike Lockwood

Comments

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Comments 851 to 875 out of 1295:

  1. Perhaps you better have a look at this paper - note the authors = for latest on solar forcing before leaping to conclusion that solar is underrated. It also appears that you do not understand IPCC figure that KR posted. Noone expects (it would be utterly improbable) that temperatures follow the multimodel mean. You might want to look at this posting at RC for further explanation. Finally, a close study of Hansen et al 2011 might be useful. Especially note the comments on model handling of aerosol forcings in light of Argo data.
  2. Well, I didn't understand the figure. Of course, because the IPCC didn't adopt it the right way from the original (Stott et al (2006b)). In the original work it is as I told that it has to be: while the global dimming the only natural caused temperature is higher than the total (anthro+nat). So what did the IPCC do, or at least the authors that wrote the chapter?! (Possibly this may be the reason why the link to the corresponding appendix is now broken in the IPCC documents. Who knows...) Ok, I'm again off topic. Back to it. The first paper you've linked is a good chance to get enhanced knowledge about another part of Sun's influence. But, if you are aware of it, this is in addition to the solar components that were used so far (TSI and SSN) and it is surely not the only parameter that may be of importance. Perhaps it will come up to the IPCC documents in the future (I hope so). As to Hansen, I will have a closer look at it. But obviously he didn't know about the global dimming because in Fig.1 (base of the whole work) the net forcing is steadily rising (except the volcanoes) what definitely would have disabled a cooling that was observed. I'll come back to it (in the right topic, of course).
  3. Joe, I cant make sense of your sentence. ": while the global dimming the only natural caused temperature is higher than the total (anthro+nat)." Its certainly not obvious to me why you think IPCC misrepresents Stott. What do you mean by "global dimming"? "it is surely not the only parameter that may be of importance" Well when you have some other parameter that makes physical sense, let us know. Meanwhile I will continue to look at the well-established, measurable physics of GHGs. If you ignore them, you cant get planetary temperature right so why do think changing their forcing is so insignificant compared to same change in solar forcing?
  4. The IPCC did run 14 'anthro+nat' models and only 5 'nat' models that possibly were independent from the other (the last is not to prove, because the description is not longer available). This cannot be the correct method because Stott had a dependence between the models. If the anthropogenic forcings sum in a negative net forcing (what is necessary for a global cooling caused by the overwhealming of GHGs by aerosols, what commonly is called 'global dimming', occured mid century), then it is obvious that the natural forcings must show higher temperatures as far as they had no change in intensity (as to the time until '63, natural forcings had always caused a warming). If you use independent models (at least 9 of the 'anthro+nat' were it definitely) there cannot be a correct result. Otherwise we would not speak about a 'primarily anthropogenic' driven cooling. That was what made it curious to me. Possible additional influences could be, as told before, for example number and intensities of flares. Each major flare destroyes an huge amount of high-stratospheric Ozone what reduces the absorption of high frequency radiation there. This, of course, has only a small short term influence, but the long term effect that results in the lagrer absorption by oceans is currently not researched, but this is currently the only explanation for the sharp rise of the OHC in 2003. In this year we've seen the most and most intense flares, culminated in the biggest flare ever measured (X28..X40, not quite sure, because the direct measurements are only possible until X17.2).
  5. JoeRG - "The IPCC did run 14 'anthro+nat' models and only 5 'nat' models... " That would be quite amazing, considering that the IPCC collected data and science from many contributors, but did not run any research itself. I suggest you check the various reports for links to the original works. You appear to feel that 'solar flares' have a much larger effect - but unless you can point to some data, perhaps a paper or two, that supports your hypothesis, preferably with some suggestion as to mechanism, it's just an unsupported opinion, not science.
  6. ... in (a) as obtained from 58 simulations produced by 14 models with both anthropogenic and natural forcings. ... The simulated global mean temperature anomalies in (b) are from 19 simulations produced by five models with natural forcings only. Well, this is taken from the caption of the figure (and I do not expect that all of the models were made by the autors). So what would you expect from one who's reading this, unable to find any description (because of dead links, just a hint that leads to Stott) and finally realizing that the graphs did match neither the original model nor the physical behaviours that are to expect? Should he say "hurray, it's from the IPCC, it's fine"? Of course, very amazing. I suggest you check the various reports for links to the original works. I would, if you can give me the list of the used models. I can't get it from the IPCC page. Regrettably there is no 'Appendix 9C' -for whatever reasons- where the underlying papers/reports would have been listed. Besides, you argued -using this figure- that the models are 'excellent'. I showed -using the same figure as well- that they are not, at least if modified by the IPCC, because of mismatches to reality and, as shown by pointing out the difference to Stott, wrong adaptations. (Of course, it's a completely different topic.) Back to topic. You are right, the flares are quite a hypothesis. But, perhaps you can give another possible explanation (eventually a paper) for the needed ~3W/m² of positive radiative imbalance. With the value of Trenberth 2006/2009 (0.8..0.9W/m²) it would need about 5 years to get the observed OHC-rise (by 6*10^22J) that obviously occured within a single year - given that the measurements are correct.
  7. JoeRG I believe the correct Stott reference is to Stott et al 2006, Transient Climate Simulations with the HadGEM1 Climate Model: Causes of Past Warming and Future Climate Change. A very similar work, although more focused on predicting than back-casting, that isn't pay-walled is Stott et al 2006, Observational constraints on past attributable warming and predictions of future global warming. Figure 1 of the second link shows a similar pattern, with current warming not explainable without anthropogenic influence. "...you argued -using this figure- that the models are 'excellent'. I showed -using the same figure as well- that they are not, at least if modified by the IPCC, because of mismatches to reality and, as shown by pointing out the difference to Stott, wrong adaptations." The models, with anthropogenic forcings included, track observed temperatures quite well. They seem to include a fairly accurate internal representation of the physics and the effects - I don't think you have any basis for stating otherwise. As to ocean warming (a completely different topic, mind you, and you should look at the appropriate links using the Search box), keep in mind that (a) our measurements of OHC are far from complete, especially at depth, and (b) circulation variances in the oceans are expected to show variations on a 5-10 year timespan.
  8. In this post /news.php?n=868#58115 Tom Curtis partially attributes cooling since 2008 to an exceptionally low solar minimum. But he effects of a solar minimum are vastly more complex than "cooling". Just two examples, the drop in solar UV (much greater %-age than TSI) creates stratospheric cooling which becomes uneven and creates blocking patterns. Those blocking patterns may induce heat waves or other extreme weather, perhaps generally globally cooling, but not simple. Another factor is high GCR which may produce more low clouds which may also be cooling or perhaps not. The other consideration for Tom is that solar effects, even the simplistic drop in TSI, are subject to the same thermal inertia as AGW (if not more). For example the entirety of lowered TSI could easily be "masked" from appearing in the GAT by a rise in SST (e.g. from El Nino). In that case (I'm not saying that happened recently or not), the solar minimum essentially only causes an OHC drop.
  9. skywatcher, my previous post on this thread applies to you as well. The "solar minimum" is not an adequate explanation of why the 2000's were not hotter. Specifically, any change in TSI is applied directly to the oceans, unlike changes in GHGs which mostly melt ice. All other solar changes change the weather which may create temporary warming or cooling. The solar to ocean connection is direct: the sun heats the ocean. A drop in solar TSI due to the "solar minimum" can easily be offset (or "masked") by a relatively small amount of SST warming, the best example being El Nino. Thus, solar mimima do not automatically create atmospheric temperature drops.
  10. Eric (skeptic) - "Specifically, any change in TSI is applied directly to the oceans, unlike changes in GHGs which mostly melt ice. All other solar changes change the weather which may create temporary warming or cooling." Eric, this is so far off that it's close to being not even wrong. Changes in radiative forcing from TSI or from GHG's have slightly different distributions (particularly in the stratosphere), but all radiative forcings affect the oceans, the ice caps, and the weather. Unless you have a reference or two to support this, I would have to consider your last post or two simply unsupportable.
  11. KR, do we really need a reference for this? The planet is 72% ocean and weighted more towards low latitudes where the sun is more direct. The highest energy portion of the solar spectrum, about 1 micron and less, are able to penetrate deeper and heat deeper. A lowering of ocean heating due to slightly lower TSI can be easily outweighed by an increase in SST due to the phenomenon like ENSO (warming from less wind, fewer clouds, etc). Thus solar minima are easily outweighed by other factors.
  12. Eric S, why would that apply to me? If you're meaning my post on the thread you link to in #858, my comments were in relation to Tamino's filtering out of 'exogenous factors', in his post How Fast is the Earth Warming?. Since 1998, we have gone from very strong El Nino forcing to a very strong La Nina forcing, and from a solar max to a solar min. Tamino has quantified the adjustments to the timeseries driven by each of these exogenous factors, and for RSS, he estimates solar forcing to have dropped 0.1C of temperature equivalent over the decade for RSS, 0.05C for GISS. Larger forcings (range of up to +0.4/-0.2) are attributable to ENSO, which is right in line with my previous understanding of the forcings. The global-scale solar forcing is perfectly real, but small, and has indeed hindered temperatures over the past decade. It is interesting to note that the 2000s were even hotter than expected on a decadal scale, given the decadal increases from the 1970s, 1980s and 1990s. It's just that relatively speaking the earlier years of the 2000s were hotter and the later years were 'cooler'.
  13. Eric, you're barking up a very wrong tree in your last three posts as well - as KR states, GHG's most certainly do not just 'mostly melt ice' - why would enhanced GHGs not impact temperatures at midlatitudes or tropics, therefore warming the oceans? If solar energy penetrates the oceans, the variations also penetrate the oceans, regardless of whether ENSO variations temporarily mask those variations. GHG-driven forcings also enter the oceans. Weather pattern changes may also be GHG-driven (creates stratospheric cooling...), and sea ice reduction-driven (altering surface evaporation, temperature and pressure patterns). Do you have any evidence for your assertions?
  14. skywatcher: the statement "The global-scale solar forcing is perfectly real, but small, and has indeed hindered temperatures over the past decade." is not supportable. Here is a supportable statement: "the small but real change in solar forcing has lowered the energy of the earth system by a small amount. That loss of energy may or may not manifest in lowered atmospheric temperatures since other ocean-related factors such as ENSO are much larger than the ocean-related solar heating/cooling."
  15. skywatcher, there are probably threads that explain why GHG energy (LW) doesn't preferentially enter the ocean, but I'll have to look around.
  16. My #865 seems to be contradicted by this: http://scienceofdoom.com/2010/10/06/does-back-radiation-heat-the-ocean-part-one/ and follow-on threads. It's going to take me a while to read through these.
  17. Ugh, my comment in 859 "unlike changes in GHGs which mostly melt ice" is wrong. See figure 3, the model results in part 4 of science of doom (please follow link above since the part 4 link is squirrely). Specifically, the heating of the ocean from changes in GHG (downward LW changes) is no different from changes in solar radiation despite the latter's deeper penetration. Now that I've corrected that tangent, please realize that my original point remains intact which is that the small TSI drop from the recent solar minimum can be easily outweighed by ENSO. That means the solar minimum does not necessarily translate to the atmosphere.
  18. Eric @867, I am unsure why you are arguing this point. Yes, a solar minimum coincident with a strong El Nino will result in a warm year, but not as warm as a strong El Nino coincident with a solar maximum. More importantly, a solar minimum coincident with a La Nina, as occurred in 2008, will result in a cooler year than an equivalently strong La Nina by itself. If the solar minimum and La Nina also coincide with increasing sulphate concentrations, either due to a large tropical volcano or industrial emissions, it will be cooler still. 2008 happened to coincide with all three, yet was still the 12th warmest year on record, and warmer than any year prior to 1997 on the instrumental record going back to 1850 (hadcrut3 global). That also means there is a significant probability that it was warmer than any year in the MWP. According to Mark Twain's famous definition, climate is what you expect, and based on our expectations (and denier descriptions), 2008 was a cold year a. When the 12th coldest year in (probably) over a thousand years is cold according to our expectations, our expectations have changed significantly. That is climate change, and GHG emissions is the only explanation that makes any sense.
  19. Tom, AGW is real and shown by La Nina in 1989 producing -.21 on UAH and La Nina in 2008 producing -0.3 or so (from the graph of 13 month running average). But that is not the point. The solar minimum being advanced as a theory for cooling in the other thread(s) is already part of the La Nina SST measurement (the drop in solar energy has lowered that measurement) so there is no coincidence of various factors (excluding the aerosols discussed on the other thread) There is simply energy and we have no really precise way to measure how much the energy of the earth changed due to those factors (and others). Simply put, by measuring the atmosphere we can make no conclusions about the effects or lack thereof of the solar minimum, especially in such a short period of time.
  20. correction: should say "-2.1" for the 1989 number
  21. Eric (skeptic) @869, we do not need to determine the change in incoming energy from a solar minimum by plotting atmospheric temperatures. We can, and do, measure it directly. Consequently we can come to conclusions about its effects on atmospheric temperatures, although the uncertainties are large.
  22. Tom, I don't think we can measure all the effects of TSI completely even if we can measure it accurately outside the earth's atmosphere. For starters the spectral changes can make a big difference (e.g. solar UV dropped a lot more by %age than TSI and although solar UV is less energetic, it has a large effect on the stratosphere). Second, the effects of TSI or any other energy changes are subject to the same constraint as GHG energy equivalents, namely that there is a thermal lag caused by the ocean. That lag is very difficult to measure since OHC is a difficult measurement especially over the short run. To reiterate my basic point, the comment that the solar minimum since 2008 should have caused cooling but didn't (or words to that effect) is unsubstantiated. It shows up a lot on other threads.
  23. Eric (skeptic) The physics of transmission of radiation as a function of wavelength is pretty solid (e.g. MODTRAN); it really isn't a source of significant uncertainty. The comment regarding thermal inertia of the oceans is also misguided. The thermal inertia of the oceans is a large part of the reason that there is unrealised warming to come due to the CO2 emissions made so far. However that doesn't mean there is not also an "instantaneous" response to a change in forcings. To demonstrate that is the case, the aerosols from volcanic eruptions cause an immediate drop in temperatures for a couple of years. If thermal inertia of the oceans buffers use from changes in solar forcing, you need to explain why it doesn't also buffer us from the negative forcing from volcanos. If you think that the comment that the solar minimum should have an effect is unsubstantiated, then again you are incorrect. Tamino has shown that solar activity has a small, but non-zero effect on temperature, see this post for details, the plot of the effect of solar activity on temperatures is below The peak-to-trough difference is about 0.1-0.2 degrees C, depending on which dataset you look at.
  24. Eric (skeptic) @872: 1) While the change in UV radiation absorbed in the stratosphere may change weather patterns because of its effect on jet streams and the Hadley circulation. It will not result in a different level of energy absorbed than that predicted by Line By Line models for that change. It certainly does not result in no effective change in the energy balance at either the top of the troposphere or the surface as you are implying. 2) The change in TSI associated with the solar cycle has been shown to have small effect on the solar cycle. The best prediction of the lag involved is 2 months (see discussion for Dikran's link). For large changes in solar output, as for example between 1910 and 1950, the lag is ten years. The reason for the difference is that the rate of change in surface temperature depends on the difference between the current temperature and the equilibrium temperature. For small changes as with the solar cycle, a small change in surface temperature will bring the surface close to equilibrium and slow further changes beneath the level of statistical detectability. For large continuous changes the disequilibrium is long lasting and hence the change in temperature detectable for a long time. 3) There is a difference between a change that induces a change in equilibrium, and a change that counters a previous change that effects equilibrium. If there is a forcing of +1 W/m^2 at the top of the atmosphere, the surface needs to warm to bring OLR and solar radiation back into balance. The heat required to bring the temperature back to balance is large compared to the additional heat gained each year due to the imbalance, hence thermal lag. But if there is a temporary reduction in TOA forcing by 1 W/m^2, the surface is already at the right temperature for the new, but temporary TOA balance. As no change of temperature is required, not lag will be present. This is the circumstance when a solar minimum is superimposed on a background of rising temperatures due to a rising GHG concentration. The rising GHG concentration requires a higher surface temperature to re-establish equilibrium. But the solar minimum reduces the surface temperature required for the temporary equilibrium, thereby immediately reducing rates of warming.
  25. Tom (874), that is interesting, but seems like an odd dichotomy. Lag of decades for CO2, lag of a decade for a similarly large change in solar output, but only 2 months for the small solar cycle TSI change. I think the answer is in your #3, that there is no lag for solar TSI cooling as you explain, but a lag for every type of warming no matter what the source (essentially the ocean absorbing the extra warmth). That seems consistent with the statement: "we should see a slowing of warming with the solar minimum" although not necessarily cooling. Your logic for why we should see that "slowing of warming" right away seems very sound, so I believe I was wrong about implying a "lag for cooling" up thread.

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