Quick thought--higher degree of resolution of investigated sites (all too many are still scattered such that it is difficult to get a good local/regional picture of the dynamics of change over time), as well as better 14-C control on sites investigated.  More detailed mapping of pre-settlement vegetation is important, as is adding to the already large data set of modern pollen (surface samples) and pre-settlement fossil pollen spectra.

In absence of delta O18, is there any other method of paleoclimate determination? Specially in case of deep sea sediments, beyond CCD, where calcareous foraminifera dissolve.

What caused the abrupt warmings at the start of the Bolling-Allerod inter-stadial and the abrupt warming at the start of the Holocene, and could a similar event occur as a result of AGW.

These are more "methods" type questions, but I think they're useful to consider.

How can we diversify our tool chest  to obtain a clearer picture of paleoclimate trends in relation to paleo- precipitation patterns for North America?

I would say that one major challenge is that statistics have come to take so much place in palaeoecology  that we sort of lost foot from the field. There is a danger of using overly-complicated statistical tools without completely understanding their functioning, the assumptions they are based on, and their limits. It might sound "cliché", but statistics will tell you what you want to hear. But what does it really mean ecologically? Does it even make sense? 

I think the greatest challenge is one - the validity of the orbital theory of paleoclimate.

After publication of D. Paillard paper ( QSR, 2015, V.107, p. 11-24) the main question is the next: "What drives climate in Pleistocene - orbital variations insolation, or oscillations of CO2?" See also discussion in Quat. Sci. Rev., 2015, V. 120, p. 126-132

Quantitative reconstructions of a global rainfall pattern from different time slices of geological time.

Yusheng Liu has a good point here. There is far more imprecision in precipitation reconstructions than in temperature reconstructions and this needs to be addressed. A lot of reconstructions are on "received water" instead of precipitation (water can come from melting ice or snow, for example). In addition, the moment in the year when precipitation is received is of major importance and annual precipitation is likely not much useful.

From a neoecological perspective I think the challenges associated with most paleoclimatological models is the applicability to near modern and modern scenarios - especially in topographically complex regions.  This is tied to issues associated with the spatial resolution of the proxies being used to infer climate (i.e., pollen studies) as well as the temporal control of the studies used to generate paleoclimate models (late - Holocene and past few hundred years). Improving spatial and temporal resolution of paleoclimate models in a Holocene/Anthropocene context could make these models much more applicable to ecology and landscape management questions related to conservation and restoration etc...

Honesty and respect for observational facts.

Resources for research.

Dear Dr. Linholm, I must follow my bias here, the greatest challenge facing paleoclimatology is to transform what we into applied paleoclimatology. We are faced with global change that will dramatically change our world, our cultures, and our lives in a matter of decades. The fact that we, as palaeoclimatologists, have not become public advocates of addressing these changes is our greatest challenge. We all know not only that there will be changes, but also what they may be. Thus far, global forums are attempting to address how we may slow or reverse these changes. However, I do not see that much progress is being made in this area. In fact, in the US, interest groups, and conservatives are actually succeeding in reversing such attempts. I believe that what we must do is prepare people for the inevitable. We must prepare agendas of change so that how we live, were we live and what we do can adjust to the catastrophic changes that we face. I don't believe anymore that it is a matter of if we will experience these changes, but when.

What I have observed in Iran the last two years, has brought me to the brink of despair. The changes have already progressed to such an extent that the international treaties on global change will be totally ineffectual in dealing with what has already happened. In fact, the changes are accelerating. We as palaeoclimatologists need to become activists collaborating with social scientists and technologists to prepare for the new reality.

Sounds grim doesn't it....well believe it or not, I am an optimist, if I were not, I would just pack my bags and find a place to spend the next few decades until the end.

Dear Peter,

Thank you for sharing your worries. However, in my mind your points of view seem to reflect ethical or moral issues rather than scientific challenges or problems. Scientists as advocates of political agenda is certainly controversial and hot topic leading to interesting further questions. I welcome variable opinions and think that scientists should express their worldviews and ideological beliefs.

Hi Markus,

I think we have enough advocates on both sides of the global warming controversy.  What would help is more proxy data from micropaleontologists and geochemists aimed at helping to verify some of the numerical models that are being used to predict future temperatures.

Markus,

Scientific and ethical issues are not unrelated. If an abrupt climate change is about to occur then it is the ethical duty of scientists to warn that it will cause disruption to global food supplies in an already over populated world.

But we do not know what causes the abrupt climate changes which happen during Bond cycles. Will global warming cause another to occur, similar to that at the start of the Holocene when the sea ice which had spread as far south as Ireland suddenly retreated to the Arctic? Will the current retreat of the Arctic sea ice lead to another abrupt climate change driven by the positive ice-albedo feedback and the conversion of the Arctic from a continental to maritime climate.

Alastair,

You may well be right but I think that science and ethics are connected only if a scientist chooses to do the connection. It is not obvious to me whether or not scientific knowledge helps to choose an appropriate approach to ethical issues. My first impression is that science is more confident with the “true or false” than “right or wrong” questions. Thinking about your example I wonder why a scientist should care and what procedures is she or he supposed to follow with the new knowledge containing potential warning to mankind! Such a message does not easily translate to political agenda or advocacy and the poor scientist is lost in murky waters. A responsible scientist then also becomes answerable to the society for these ethical choices and not just the science. I am looking for paths to find justifying reasons for action.

C.T. Schafer:

Do you mean that scientists should not act on global problems at all and just continue collecting more and more data.

Marcus,

if a scientist is in a theatre and sees smoke coming from an exit door, is it not his duty to shout "Fire!"

We know that NH temperatures jumped by several degrees Celsius within a decade twice in the last 15,000 years, as the planet warmed at the end of the last ice age. If a scientist knows that, then is it not his duty to warn the general public that this could happen again, as we warm from an increase in CO2?

I agree with Alastair that scientists have a duty to warn the public of the environmental, economic and social issues related to climate change. However, it is not an easy task and, if done in the wrong way (even though with good intentions), it might result in a result opposite to what was expected. See this blog post for an interesting discussion on that:

https://medium.com/@kabrass101/why-u-s-environmentalists-lost-the-climate-change-fight-c67e00c462a8

Hence, I think we need both more (and better) data and more (and better) advocates. Climate science is much complicated and climate scientists are the ones with the best understanding of it (or at least they should be!). They need to ring the alarm first and clearly, but then scientific columnists and journalists need to jump in to help make sense out of all this and make the info digestible to the public, all political views combined.

acquiring paleoclimate data is a great challenge