Comment on esd-2021-67

This paper is a tour de force compendium on the latest scientific results relevant to understanding recent and future climate change on the Baltic Sea Region. The authors and the Baltic science community are to be commended in taking this on in a way that builds on and updates the two previous BACC assessments. It is particularly effective that the assessment is linked with the efforts of HELCOM. It sets a high standard for climate change assessments for regional seas in other parts of the world.

Thank you very much for the thorough review and excellent comments. We will follow your suggestions and will revise the manuscript accordingly. We are impressed by your review work.
This summary paper brings together and depends on the results of nine specialty papers or BEARs, which I have not reviewed. Nor have I been charged with reviewing the consistency of the summary with the BEARs, but trust the authors to ensure that consistency. The construct wherein each environmental variable is treated under present climate change, future climate change, and knowledge gaps and research needs results in some redundancy. This could be alleviated somewhat by reference to the corresponding previous section without repeating narrative and references. The section on concluding remarks does help bring these all together.
Within Baltic Earth, we performed prior to submission an internal review with two reviewers that were not involved in the BEARs to guarantee consistency.
The list of knowledge gaps and research needs is rather daunting, and rather depressing as it seems that virtually everything uncertain and unknown, and equally so. Clearly, this is not the case. Concluding that section with a brief consideration of the knowledge gaps and research needs that are most critical to determining the future Baltic and are most potentially resolvable with concerted research would be help.
We will add a concluding paragraph.
The words uncertain, uncertainty, and uncertainties are used some 104 times in the paper, and often incautiously. Frequently, there are better terms to describe the nature of these so-called uncertainties. They may be a result of inadequate knowledge rather that inherent uncertainty or they might actually be deep uncertainties. In particular, when future changes depend on steps society might take to limit greenhouse gas emissions, these seem not so much as uncertain but yet to be determined. Some finetuning of the uncertainty language would help.
We will fine-tune the uncertainty language.
While, the paper indicated it will follow the terminology used by IPCC concerning degree of confidence in statements, as it does in section on key messages, it is not as very careful when it comes to the use of the term "likely" some 64 times and doesn't seem to differentiate among as-likely-as-not, likely, very likely or virtually certain as per the use of likelihood terms in IPCC parlance. Similarly, the term unknown is used quite a bit, without differentiating among completely unknown, largely unknown, not fully known, or incompletely understood. These might be more accurate descriptors in places.
We will check and possibly be more specific in the revised version. However, for many variables the information about confidence levels does not exist at the regional scale because large ensembles do not exist. We will explain this fact in the introduction and modify the definition of our terminology.
While this paper was developed prior to the release of the IPCC Sixth Assessment in August 2021 it is appearing after this release. Virtually all the literature cited used earlier GCM results, although some results based on CMIP6 models are discussed. It is impossible and unreasonable that this paper attempt to incorporate or compare Sixth Assessment models and conclusions in any great detail, it would be useful if the authors wrote brief comments about the extent to which conclusions might be affected by the new IPCC assessment, perhaps after section 1.5.5. My sense is that they wouldn't dramatically affect the conclusions. Recent literature (e.g. Hausfather and Peters, 2020, as cited in this paper) makes the point that the RCP8.5 pathway and the associated 4°C warming during this century is highly unlikely to occur and, in fact, the IPCC AR6 essentially admits this. Something between RCP7.0 and RCP4.5 is probably the maximum warming without substantial mitigation. Perhaps this point can be made more strongly in this paper. In fact, it would be informative to mention in key places where mitigation measures would affect key climate drivers, if and as society significantly reduces emissions and the use of fossil fuels (e.g. this would affect N deposition, shipping, plastics, etc.).
Following the comments of both reviewers, differences between CMIP5 and CMIP6 and how these differences may affect our results will be discussed.

Specific comments:
What are the current Baltic Earth Grand Challenges, a listing or, at least, a reference (line 128).
We will add the Grand Challenges and a reference.

Farther (not further) poleward as this refers to distance (line 2290)
Will be corrected.
The future scenarios for shipping do not include a future where the use of hydrocarbon fuels or at least emissions of CO2 are greatly restricted to meet GHG reduction requirements. Could the authors speculate what this might mean? (lines 2353) This is a valid point. The current available studies for shipping in the Baltic Sea do not include the strong fossil fuel emission reductions IMO is postulating (i.e. 50% less greenhouse gas emissiones by 2050). The IMO secretary-general states in the foreword of the Fourth IMO Greenhouse Gas Study: "The Study demonstrates that whilst further improvement of the carbon intensity of shipping can be achieved, it will be difficult to achieve IMO's 2050 GHG reduction ambition only through energy-saving technologies and speed reduction of ships." New synthetic fuels are needed or alternative propulsion. We are just running our CTM for scenarios addressing those requirements. Results will be available in spring next year, earliest.
In a synthesis paper for Shipping in the Baltic Sea, we will show that under current legislation and quite strong energy efficiency assumptions (stronger than the EEDI from IMO), CO2 emissions in the BS will drop down to only about 78% in 2040 compared to the value in 2014. Other measures are needed, while the use of LNG as fuel is a good solution for reducing air pollutants like NOX, SO2 and PM, CO2 emissions remain considerable. Methane slip during transport and operation can even compensate for the reduced CO2 emissions. This paper will not be published in time for this summary.
We will close the paragraph with a more general remark (line 2369): "The pollutant concentrations reported in this section may drop to yet not known lower values if the shipping sector is (partly) successful in meeting the IMO target in greenhouse gas emission reduction of 50% by 2050. This reduction is only possible if low-carbon alternative fuels will be introduced, employing a high energy efficiency as it is already considered in scenarios used in Karl et al. (2019a) will not be sufficient. The new fuels will also lead to altered emissions of pollutants." In this section, is the use of "likely" versus "very likely" consistent with IPCC? (line 2372) We will change "may have an influence" because it is difficult to state this cause-andeffect relationship for all systems of the hydrosphere.
This paragraph refers to both mitigation measures and adaptation measures. This is confusing in light of the way those terms are used in climate change assessments. (lines 2439-2244) We agree and we will use "anthropogenic measures".
Here again adaptation and mitigation are both used in a somewhat confusing way (lines 2450-2464) We will rephrase the paragraphs.
This sentence is unclear. (line 2556-2557) We suggest the following alternative formulation: "The authors argued that a more comprehensive assessment of forest management as a strategy to achieve the goals of the Paris Agreement should go beyond the reduction of atmospheric CO2 and, thus, the reduction of the radiative imbalance at the top of the atmosphere. They suggested…" To which models are you referring? Do you mean under both RCP2.6 and RCP8.5 emissions pathways? (line 2568-2569) We will rephrase the first paragraph to clarify the changes under the various RCPs.
There is a need for a reference for this paragraph, I assume it is BACC II Author Team (2015). These conclusions about decreased pH are contradicted somewhat by the previous discussion in section 3.2.5.7.2. (lines 2878-2883) We will add the BACC II Author Team 2015 reference and clarify the apparent contradiction.
The concept of retreat of marine species may not be clear for readers unfamiliar with the Baltic Sea, perhaps this can be more accurately stated as reduced penetration of marine species into the Baltic Sea. (line 3024) We will add new text in line 3024 … many species in the Baltic Sea. Because of the projected decline in salinity, a reduced penetration of marine species, such as bladderwrack, eelgrass and blue mussel, into the Baltic Sea has been predicted (Vuorinen et al., 2015). A large number of other species is affiliated with such keystone species, and species distribution modelling has indicated that, e.g., a decrease of bladderwrack will have large effects… Here again, it seems to be assumed that shipping will continue to depend on the use of fossil fuels. (lines 3236-3237) Isn't it more accurate to state that how these practices will change in response to climate change is yet to be determined? (line 3245-3246) We will rephrase this sentence to emphasize the research needs in this respect.
Perhaps state that there is YET little direct evidence THAT THIS IS OCCURRING? (lines 3261-3262) Thank you, as for the previous comment, we will rephrase the sentence.
It is mentioned earlier that warmer temperatures should allow the establishment of more nonindigenous species. This bears repeating here. (line 3263).
We will add the information.
Won't microplastics also be greatly affected by societal decisions about the use of plastics, in part influenced by efforts to decarbonize? (line 3283-3285) Thank you, that is a good point; changes in the use of plastics and regulations in response to the problem can be expected but the effects are uncertain. We will add a short sentence in this respect.
Should the authors continue to use Celsius rather than Kelvin here? (line 3399) Yes, we decided to use Celsius in the entire manuscript will change here.
. . . strongly affected by whether warming is allowed to proceed to the point of destabilizing Antarctic ice sheets. (line 3605) We will rephrase the sentence.
What does it mean to have low confidence in a statement that changes could not be detected? (line 3750) The confidence level refers to our knowledge about the change in large-scale circulation and not to the specific statement. We will explain the confidence levels better.
If this trend is almost statistically significant, why isn't it medium confidence, just less that a 95% threshold for high confidence? (line 3829) We agree and change to medium confidence.
Why is there only low confidence in the statement that larger runoff would lead to larger nutrient inputs? (lines 3920-3921) We agree and change to medium confidence.